You can start the course for as little as the cost of a trial lesson (£90) though i think there may a slight incentive to pay for the whole course up front (£5625 - though you will go over this with books, equipment, away landing fees, extra lessons etc). May be best to "pay as you go" - all prices are on the website.
As i said, the airfield is 20 minute walk from Denham station. Denham is a 30 minute train journey from Marylebone (west/central London) and there are other stops throughout NW london on the way.