Regarding Ravenair and paying upfront, it is an option but they don't expect you to do so, least not when i used them last year. You could also pay as you go, or open and account with them, I held funds in one which they would access for payment, it also got me a small discount (I believe you had to open the account with £1000 and then keep it topped up above £100). I would strongly advise you consider the other two payment options rather than the pay all in one option which like others i would advise against.
I'm currently doing my theory with them. Also check out...
http://www.merseyflight.com/
Mersey flight is a new school but seem a good set up and you wont have the trouble of short term hour building instructors like I had with my previous school.