Actually, here's my two cents on this: while I think the capabilities of the British Army are massively overrated given the few times they participated in operations with someone else's army (other than Spain's because the Spanish were starting from the ground up) on the continent...armies are not a magical solution to everything. Even for l'Empereur.
First of all, Napoleon would move relatively slowly because as unprotected as England was, guns take time to travel around.
Second of all, even though he magically had his (realistically a few days while the British navy was tied up somewhere by the ASBs) to land, the situation wouldn't persist forever.
The British navy will return and cut him off.
That means that Napoleon would have to occupy port cities, one by one, to deny them safe anchorage.
The British might have ample time to raise militia and reserves. While on the field they might well get smashed, they would make taking every location more costly, being more useful in defense.
French army would also land without much supplies. They would need to capture food and stores before they can conduct operations.
So since we keep talking about Prussia: how long did it take Napoleon to beat Prussia in the 4th coalition? About a year. This is with Russia and other supporting Prussia, but with Napoloen having uhindered supplies.
The situation is reversed (England alone, Napoleon needs to forage), but all of GB falling with determined resistance may really take that long, in the worst case.
Which gives Britain's European allies opportunity to try their luck at getting back at the French on the continent.
As far as Prussia is concerned, it took only a few weeks to annihilate the prussian army. What took a year is the total submission and surrender of Prussia which required to defeat the russian army.
But you have to take into account is the geography.
As I previously mentioned, it was at the time almost impossible for the French to cross the Channel given the superiority of the RN.
But if we are in WI in which the French get their time to cross, then they cross. They manage to cross with the logistics that had been gathered for example at the camp of Boulogne. Which means horses, food, ammunitions, ...etc.
And, coming back to geography, once you are in England, there absolutely is not the same topography that favours defence and guerilla warfare as there is in Spain.
England is not either Spain in cultural and economic terms. It of course had a strong patriotism but there was no religious fanaticism. England was the most advanced and richest country of the time. When you are the most advanced and developed country, you don't inflict on yourself the same damages and violence as the Spanish did.
The people accepted to fight as long as victory was almost certain. But if the French crossed the Channel with 200 000 troops, what would be their reaction ? Would they fight for a corrupt oligarchy ?
There was a strong feeling feeling of solidarity between the nobility, the trading-financial-business groups, and to sum-up quickly, the ones whowhere closely enough associated to them. But did they represent more than 10/15% of the population.
And do you think this ruling class was prepared to let its properties destroyed or taken by the invader and hide I don't know where ?
If a big french army had landed, I a not even sure Napoleon would have needed to take control of all the main british harbours so to starve the RN. The english ruling groups would have almost unanimously sued for immediate peace. They would not have said "Let's burn everything down, win that damn war. We'll build it from from scratch if and when we defeat those damn frogs."
I guess it would rather have been : "Let's save/spare as much as we can."
Other questions :
- How many guns where there in England at the time ? Did every citizen have a gun ? This was England, not America.
- What do you think would have been the reactions in Ireland when they learnt about the french invasion ?
- and the reaction in Scotland ?
As far as the british land army is concerned, it took very specific conditions and a very long time for Wellesley-Wellington to train a good and small land army : the spanish and portuguese quagmire and narrow passes.
Where do you build build the Torres Vedras fortifications in England ?
You ask about food ? Well, the english population needeed to eat to. An invader lives on the country as far as he needs to.
If there were 200 000 french troops campaigning in England, don't you think it would very seriously damage and disorganize the food supplying chain necessary to sustain life in towns 2 centuries ago ?
Contrary to Russia, they had nowhere to flee.
So if (and I do agree that it is an enormous if) the french army landed in England, the war is lost for England as certainly as Achilles was almost invincible but would die if seriously wounded at the heel.
And since England and the british ruling class were all but suicidal, they would have sued for peace. They would have kissed goodbye Ireland, probably Scotland too, a good part of their Navy and of their colonies.