We had the same questions last year and we found that all the british companies seem to have an agreement to charge the same price for attraction tickets.
I was told to wait until we got theire and went to one of the Walmart stores which had a travel section and bought our Disney and Flextickets (Universal Studios etc) there and saved about �150 and the tickets were identical to the ones you could buy over in the UK.
A word of warning though. Don't buy part used tickets at service stations or the like, because when a new ticket is first used the user has to sign it on the back and when you enter your first park you place your index finger on a scanner and then whenever you go to another park you put your ticket in the barrier and then put your index finger onto a small glass panel and the computer verifies that it's the same person who used the ticket on the first occasion.
My wife always kept our 4 tickets in her purse for safety, and when she handed out the tickets for us and my son and I got each others tickets and when he went to go through the barrier it flashed red and one of the Disney employees came and sorted it out and said it was a very common mistake