[Rainbow Six]

I think any large scale British presence depends on whether or not you have the 1990's era withdrawals happening in your game world. If you don't and you posit that the UK maintains a Regular Army Battalion in country right up to the start of the Twilight War it's feasible that said Battalion then stayed in situ - whilst it could be moved prior to Autumn 1997 you could argue there weren't the available logistics to do so and then post autumn 1997 it simply isn't possible.

IIRC the British Army orbat in the NATO Vehicle Guide and the official UK Survivor's Guide omits a number of Infantry Battalions - from memory several Guards Battalions and 1st Battalion Worcestershire and Sherwood Foresters - so possible one of them was the resident Belize Battalion

Or you could go with the Regular Battalion being replaced with a TA Battalion as a backfill measure. As I said earlier, I think that's perfectly plausible, but the powers that be in London would have to accept that would leave a shortfall elsewhere - so you're robbing Peter to pay Paul.

If you posit that the 1990's withdrawals did take place - e.g. if Belize and Guatemala reached an accord as they did IRL - I can't see any way that there would be any sort of subsequent reinforcement at the start of the War if the Guatemalans started sabre rattling. Given geopolitical considerations elsewhere, once the resident Battalion is gone it would stay gone - troops would be badly needed elsewhere. You might see a token force - as I said earlier, some sort of reinforced Infantry Company, Regular or TA - to augment the BATSUB staff but that would be it.