Sounds good to me!

You put your finger on it that the chief problem is whether the girlfriend will like this lifestyle.

I would say, don't commit yourself to leaving the Caribbean at this point. You can spend any amount of time there, girlfriend will find the sailing lifestyle much more amenable than in England. Decide later (certainly not before the next start of the next hurricane season) whether you are ready to move on. If girlfriend doesn't like sailing in the Caribbean she won't like it anywhere, but Plan B sounds pretty good too.

Keep it nice and easy for girlfriend this summer ... stick strictly to the Solent or similar sheltered waters, fair weather, maximise her enjoyment and don't overdo the 'training'. A hard open-water cross-channel slog at this point risks deterring her fatally.

With your budget and eventual grand plans, although s/h boats are cheaper in the Caribbean I'd aim more in the 36' - 38' bracket, get a really well-found boat and hold a fair amount of cash in reserve for doing it up. Don't commit this money until you've tried out the boat and found what needs replacing!

You say you are a YM? You do mean full qualification not just theory? If so you shouldn't need any more. If not, do the practical! For the girlfriend, once she's had a bit of practice a 'competent crew' course away from you at a reputable sailing school may well increase her motivation.