No matter how good a deal may be, you should never pay up large sums of money at the begining. The only exception is if you pay a block of say 5 lessons to keep you in credit for the next week, or to cover test fees (which may be required in advance)
Paying large sums of money in advance carries the following risks:
1). The FTO going bust.
2). The FTO not being up to your expectations.
If you stopped your training due to reason 2, then how easy could you get your money back ? Even if you could get your money back, you could suffer an unreasonable delay which could waste valuable flying time (especially during the summer months in the UK).
You are secured with a credit card, but only if the FTO is in the UK.