Don't pay in advance.
Tons of good advice above, particularly Zulu Alpha's.
I did exactly what you're thinking of... US then UK... not my plan exactly but I didn't get all the hours in the US - I was there 3 weeks. I would assume that at your age (no offense!) you won't do it in 45 hours. That's fairly normal. I think I took about 60, maybe a bit less.
I'll reiterate Zulu's comment about a busy airport. Get some independent opinion. I'm sure any school hungry for business would say "sure! we're busy!". I moved from a very rural field to Kissimmee near Orlando, which was hectic at times but meant I learnt alot about one of the more challenging bits of flying.
One very big thing though... you say combining it with a holiday, and "we" ? You won't be able to rack up the hours if you've got other people to think about. The US intensive thing is utterly exhausting. Three or four hours flying in a day - when you are learning - will more than likely leave you shattered.
If you want to combine this with a family/partner trip then I'd say you are looking at 2 hours a day flying max.
And one last thing. Don't pay in advance. Have we said that yet?
