Pretty much agreed.
The United Kingdom's agreements with France were exceedingly ambiguous, a situation that was very deliberately engineered by Sir Edward Grey, the Foreign Secretary, in order to ensure that the British government could essentially act as it saw fit.
The Treaty of London that guaranteed Belgium's neutrality was not, as it is often crudely misrepresented, a British guarantee for Belgium. It was a multinational guarantee for Belgium. The UK might have felt some extra responsibility because it had hosted the treaty, but there were many Britons of the opinion that the UK had no more duty to defend Belgium than any of the other signatory great powers. Ultimately it is my contention that the duty to Belgium was a major part of the British justification for entering the First World War IOTL, not a major part of the British reasoning.
Another major part of the UK's justification for going to war against Germany IOTL was that France had (rather foolishly) been so sure of British support that they had coordinated their naval strategy with that in mind, thus leaving their western coast virtually undefended in the expectation that the British Grand Fleet would take that area; thus, the argument went, the UK had to protect France, rather than leading France to believe in British intervention and then betraying the French. This argument will hold just as fully in this scenario as it did IOTL.
As for the United Kingdom's actual reasoning for going to war (mainly the growing economic power of Germany and the idea of a two-pronged problem: either Germany and Austria-Hungary win, in which case they're hostile to the UK, or France and Russia win, in which case they're hostile to the UK for betraying them by not entering the war) none of it will change.
So in my opinion the result of this would be that the UK's justification for entering the war would look much flimsier but it wouldn't stop the UK from entering the war. The major effect, in turn, of this would be that it decreases the probability of the United States entering the war against Germany.