Since you can't get a PPL until you are 17 (solo at 16), its going to take you more than a year to complete. You really don't want a flying school to be holding on to all that money for that length of time.
Ask them about "Pay as you Go".
I don't know anything about Booker, but it sounds like a Pay As You Go scheme, but instead of paying each time you finish a lesson, you charge up an "account" in advance. By paying through the account they are putting 3% bonus money into the account when you put your money in.
Just don't top it up too much / keep too high a balance as you'll end up with the same issue as a pay up front scheme.