One of the major drawbacks to learning at Denham (which they may not tell you !) is the distance to the training area. Because you are underneath the Heathrow zone you have to fly right out towards aylesbury on every flight before you can climb. This adds a considerable cost as you are paying an extra fifteen minutes or so flight time to every hour's training. You may be better off to drive a little bit further and fly from an airfield which has no such restrictions. I think £125 per hour is pretty steep and I bet the instructors don't get paid £40 per hour (but I may be wrong, of course). Depending on where in London you live, you may want to try Stapleford, Booker, Little Gransden or Bourn. The airfields to the south are a lot more expensive. I echo what everyone else has said - go visit a few airfields, get prices, compare, meet people, talk to students and instructors, take a trial lesson. It's a big investment you're going to make so it pays to research it well. Enjoy your flying.