UK Flight Training run the JAA course in Longbeach, California. I believe you can also train at Rainbow Air althought they use UKFT examiner for the final flight test.
You dont have to learn to fly in the UK to fly in the UK as someone said earlier - its not 'all different' and if you learn to fly in CA you can find any conditions you want to fly in from blazing sunshie to fog or mountains ranges complete with ski resorts!
The only thing you will have to get used to on return is a different style of RT but that doesnt take long and you for what the differences are you could cover them with a bit of ground school and then polish it up on your check flight for whichever club you join on return.
If you want to know any specifics of either place then feel free to mail me.
Julian.