As the years went on after 1865, the UK imported more and more grains thru WWI. UK farmers couldn't compete with cheaper US and Canadian grain.
Doesn't have to be full fledged famine with the Poors dying in the street, for higher food prices being a real political problem.
Especially in the context of this being one of several prices the masses pay--including higher taxes, greater liability of being drafted, pressed or otherwise induced into risky and disruptive military service, all in the aid of a "classes" versus masses antilabor agenda in the form of actively aiding the CSA against the USA.
How this plays politically depends on how the Americans are perceived as being culpable or not in the war
by the lower class politically active sectors. If the US is seen as having rashly and greedily attacked such British interests as say Canada, quite a lot of sacrifice might be endured. But if the USA is seen as finally moving against slavery only to have their own government jump in to protect the slavers, and violently attack the USA to do it, I think a US counterattack against Canada will be seen in quite a different light--and so will any and all of the privations workers must suffer for the sake of the war. If the US signals that its moves against BNA are defensive and return of lost territory is still possible, then the continuation of war against the USA in favor of the CSA becomes less politically tenable. If the Yankees also toss caution on the subject of abolition to the winds and declare emancipation, even in the hypocritically limited sense of Lincoln's OTL Proclamation, the anti-war cause gains even more legitimacy.
Of course at this date this is not a matter of the masses voting out the current government; most working people opposed to aiding the CSA have no vote for Parliament. But that too is an issue, and a reason that the USA is seen in a positive light by many of these working people. If the classes that do have the franchise will not listen to rumblings of the beast and move to placate them somehow, they might be facing a resurgence of militant Chartism or something worse.
Being conciliatory does not even require the elite interests to give up everything they did OTL; it involves negotiating with the Yanks and leaving the CSA to fend for itself--conceivably, having gone to war and then agreeing to stand down again, they can fancy-dance ongoing diplomatic recognition of the CSA past US effective protest, on the principle that the best thing about being hit on the head with a hammer is it feels good when it stops--mere formal reception of CSA envoys at the Court of St James might seem not such a big deal if Britain goes from actively allying with the Rebs to just sipping tea and nodding without doing anything helpful at these envoys.
They have alternatives to placating the masses; they can try to crack down. Certainly if the USA does anything against Canada to retaliate, no matter how restrained, some Britons will find that unforgivable, and in the middle of a war it is generally possible, at least initially, to impose quite a lot of reactionary repression. It is easy, early in a war, to get jingoism to drown out class interest politics. This is a major reason lots of stupid wars get started after all.
But...over time people tend to sober up if the war seems to be drawn out and not so glorious. Trent War Anglo-wankers all seem to assume it will be quick and glorious for Britain, and perhaps it might be. But I think the USA will be more tenacious than OTL even, if Britain throws her weight around in America, and then it will not be so quick.
It is impossible to say definitively whether the outcome leads to such a gross erosion of ruling class legitimacy despite the most effective forms of repression the classes can muster that mass politics seizes control in a violent rupture, or if in fact the working classes can be kept in check handily and indeed OTL liberal progress is checked and reversed. It is worth noting one figure in favor of quick and easy recognition of the CSA, and quite possibly then liable to favor other escalations of worsening hostility with the USA, was Gladstone; if opposition to the war puts him on the side of reaction across the board, quite possibly this signals a general hardline attitude against all progress toward democracy in Britain, and the development of quite a different set of norms around the turn of the 19th into 20th century across Europe.
It seems bloody typical of the people who support the idea that Britain can easily enable CSA survival, that they disregard the importance of privation, even perhaps just moderate inconvenience, but still privation for no good reason from the mass point of view. They are just working people after all, how much can they even feel pain? It's not like they are real people who matter!
As I was writing the above passage and wondering if I was perhaps being too harsh or needlessly unkind, someone came along with yet another comment disregarding the intelligence of the common people in classic elitist fashion. This is the attitude one needs to be at all comfortable even contemplating the survival of CSA on just about any terms, so it should not be too surprising that elitism is the mindset.
Any mass uprisings that lead to brutal reprisals will cause a political firestorm in Britain. After all, it’s directly their fault that these uprisings happened! I could see the current government being brought down over this and the next one distancing itself from the CSA, in all honesty.
Of course this is only true if Britain actively twists the USA's arm--it can happen to a limited degree if HMG appears to have encouraged the CSA and discouraged the US to fight for reunification, but it only becomes a serious factor if in fact Britain is engaged in war with the USA, which is not a necessary condition for the CSA to be independent! If Britain does not come to open war with the USA there will also be no interruption of grain shipments either. Indeed I expect the "classes" of Britain to pay a political price if the CSA endures in any circumstances whatsoever, but if the US "simply" decides on its own not to try for reincorporation of the seceded states, much of the moral onus for any monstrous deeds in the South will fall on the northern government, for renouncing all options to aid the slaves to avoid a war; it will demoralize any republicans or democratic reformers in Britain, at least those relying on working class solidarity, along with discrediting the moral credentials of the elites who favor supporting the CSA for various expedient reasons.
CSA survival on any conditions at all tends to make the TL a crapsack 'verse. It was a scummy enterprise by scummy people, naturally these people prevailing is a bad thing with bad effects all around.
You know what, demonstrate to me that there were food riots in Britain in 1868 and then we can have a conversation
Because the total supply per head for the year 1866-1868 dipped to 4.8 bushels or less compared with averages of well over 5 bushels per head even topping 6 bushels at times during the ACW
Link
So by your logic: political crisis...where is it?
You know what, demonstrate to anyone that these are equivalent circumstances! This argument in classic CSA apologist fashion shows a disregard for the agency of common people as political actors.
It is very very relevant whether a hardship emerges from the egregious misleadership of the powers that be, or not. For the elites of Britain to make a decision to go to war with the Union puts any consequences that reasonably can appear to emerge directly from that decision in a very different light than the same objective hardships appearing to emerge from circumstances not in anyone's direct control.
As noted, the drop in practical availability of foodstuffs would, in a Trent war scenario, be associated with other hardships at the same time due to the same obvious cause. Any statements about how people react need to integrate the whole situation, not treat people as mindless robots who respond to a given input with a fixed output without regard to general circumstances. Context is everything. Much worse privations might be endured by Britons who felt it was a necessary war, and much milder ones cause for major political upheavals if they step from a frivolous error.
This is why we can't have a generic vague "Trent war" scenario without specifying the details.
And why I prefer for this thread to focus on a simpler if arguably no more probable one in which the USA simply does not choose to incur the grave costs and risks of fighting the secession...and thereby accept incurring the consequences of that decision for inaction instead.