from what I can tell the mnarket is very large - but tailored for the Americans, just like St Thomas, Aruba and St Kitts
more of these are starting to lean towards the British market, but that takes time when competing with the East Carib dests like Barbados, Antigua & St Lucia, as well as the mass markets of Dominican Rep. Cuba and Jamaica.
I don't think a UK-SJU flight would work. The only selling point in this country is it's cruise base, but then the majority of those cruises are aimed at Americans too - BGI is more UK cruise led.
I think over time though some of these markets will start attracting some UK flights - I've already heard a few scheduled carriers are looking at St Kitts and St Maarten - I guess this would either be BA or Virgin, however with bmi now from Manchester it could be those guys - I don't think BW could do it due to lack of slots in London.