If you do decide to pay up front, pay by credit card so if they go bust part way through your training you can get the rest of your money back. Some very well known and reputable schools have gone bust with little warning, leaving students out of pocket.
As others have said - few people complete their PPL in 45 hours, and many of them do it as an intensive course (so less re-learning). Budget for at least 55...
Written exams are likely to be £25-£30 each, with RT practical perhaps £90 and skills test a lot more, although there isn't a fixed price. If cost is part of your calculations, perhaps worth asking the schools in advance? However, they wont add up to much more than a couple of lessons. Landing fees add up too though. When learning the circuit you may have 8 landings in one lesson. If you are paying £10 per landing...