Never pay up front. There a good reason why I'm repeating what two others have already posted - it is rule number one of learning to fly.
I'd also recommend you fly the AVGAS aircraft, for the reasons stated above. I did part of my training many years ago at Wycombe, with what is now Airways Flying Club, and White Waltham has a long established and good reputation as an airfield, so no recommendation either way from me.
Gertrude is also correct regarding minimum hours. I worked at a flying school for over two years and he's right.