A prid pro quo for the Spanish ditching their support The revolutionaries.
I can go with that. As for what you said about the "TTL war of 1812"...I'm not so sure, though I could be wrong. Especially if it's the early 00's, but generally in this decade the UK was far less committed to the continental war with Napoleon than in 1812, which was iirc why the Americans struck when they did. Britain has very few land commitments at this point and therefore would probably be able to mobilise the troops quickly to quickly subdue the USA. In addition, in OTL the Americans failed to attack Canada and then saw their capital burned, I suspect a war on two fronts would go badly for them. Further to this, the south has become an area of concentrated loyalism, which would likely lead to the fielding of a number of militia regiments in British Southern North America to at least fend off an invasion there, if not to substantially bolster a British invasion of the USA. Personally, if I had to call it I'd say the USA would request an armistice after 12-18 months and the UK would take the opportunity to attempt to bottle them in so they couldn't expand into the west, maybe by demanding that the USA cede their claims to the Tennessee Territory and part of Kentucky with a consolidation and recognising of British control of Illinois/Winsconsin, though this is pure speculation.
It is, of course, entirely possible that there are factors I haven't considered, though...