Who do you consider the British Establishment? OTL Grey represented 50% of the total British Cabinet members prepared to go to war without a German invasion of Belgium. Churchill representing the other 50% of the Cabinet members in favour of war, was in shadow discussions with the Conservatives fearing a scenario where the British Cabinet would remain neutral even after a significant violation of Belgium neutrality.
The British hawks (the aforementioned Grey and Churchill) had premised their position on a German invasion of Belgium, the absence of which would result in them being discredited. Belgium was a pretext for the hawks, but they only represented a small minority of the British Liberal Cabinet. In contrast, Lloyd George who was a de facto leader of a block of 7 cabinet members on this matter (doves and non-interventionists) was already on record as stating the vital important of the channel ports to British national interests.
I apologise for the lack of precision and accept the rebuke: in this context, I am really using establishment as a (potentially inaccurate) cover-all term for the Pro-war Liberals and the Conservatives. Recent reading has led me to believe that Grey and Churchill were adamant for war, backed up by Asquith and Haldane (Minister at the War Office) and possibly Reginald Mckenna (Home Sec.). Whilst they were outnumbered by Lloyd George et al. the hawks crucially held the major offices (in which I refer to the offices from which military and diplomatic information would be readily available, I am aware that Lloyd George was Chancellor of the Exchequer). Grey obviously was Foreign Sec., Churchill was First Lord of the Admiralty and Haldane was War Minister. The others in the cabinet were increasingly perturbed by the fact that they felt they did not have all the available information and that the hawks were trying to manoeuvre the cabinet towards naval action, from which they would later extrapolate using the BEF in France, under the guise of upholding Belgian neutrality.
Why do people always insist on the personification of nation states - "Britain" does not have an opinion. While the British hawks did not have the power to drag Britain into the war, they did have the reigns of British diplomacy and naval policy. So the British hawks led Britain to the cusp of war, but could not have got them across the line without the Germans being in Belgium.
The 1839 treaty had served Belgium and British interests for over 70 years and Britain was not in a position to dictate domestic policies (i.e. military priorities and spending) to an independent Belgium. If Britain had seen the a genuine German threat on the continent, perhaps they could have at least had the framework for their own continental army?
"
Britain" in this context was meant really to refer to successive British governments of the 19th and early 20th century seeing it as their interest to have Belgium both neutral and undisturbed, as a long-standing objective of the country's foreign policy, it is nothing more than broad shorthand. I will concede that I went too far with what Britain could have asked of Belgium in terms of Belgian domestic policy.
However, I will clarify, I hope, that not only is it somewhat obvious that the British government of the time did not consider, and that their actions belay the notion that Belgium's neutrality is what their concerns were genuinely about, instead they were using violation of said neutrality as a means to enter the war. I also wanted to convey that, if that British government or any successive government had really believed it to be important to preserve Belgian soil from foreign attack it could and perhaps would have pursued greater obligations from all of Belgium's neighbours in treaty obligations, probably long before that war ever took place.
As for the Treaty of 1839 being in British interests, I would have to disagree to an extent. Beyond the creation of the nation itself, particularly to ensure none of her neighbours could get access to her coastline (and Antwerp) from which they might threaten Britain, I cannot see how it contributed to, or was utilized by, any subsequent government in any way that I would describe as pertaining to British interests. Late-war versions of the September Program actually reference the complete annexation of Belgium and the militarisation of Antwerp as a direct threat to Britain after a German victory, something I highly doubt the German Establishment (Generals and Polity) would have considered before the war. In 1870, the Treaty of 1839 was useless, or at least Gladstone considered it as such, hence why he made his own arrangements with respect to the Franco-Prussian war and also why I feel the Germans were well within their right to refer to it as a 'scrap of paper' when they marched through in 1914. Grey couldn't even convince the cabinet that it obliged Britain to get involved in 1914.
As for the framework for a continental army, I am headed back to Niall Ferguson's point on this: that Britain's alternative, viable option other than neutrality was spending a large amount of resources before going into war preparing a large army and appropriate tactics for such a conflict, whether that build-up commenced before or during 1914. Conservative politicians had actually been pushing for greater increases in the war budget and such for about a decade beforehand in anticipation of a war in Europe. Indeed perhaps if it was agreed that the Treaty of 1839 or any other agreement made
did bind Britain to uphold Belgian neutrality
with military force if needed, then governments would have had the grounds on which they could build such a force.
Personally, even if British pre-war policy had prepared 4 million crack troops instead of 400,000 to land in France in 1914 she would have been better staying out of the whole blasted thing, looming over all the losers and victors who would have all lost hundreds of thousands if not millions of men with a navy and balance sheet they could not defy.