Hi,
as an ex-resident of Uxbridge I would recomend you try The Pilot Centre in Denham.
That's where I completed my PPL (ah..the instructors are still undergoing therapy!
). It's very handy (10-15 minutes away by car), nice little airfield and a great bunch of people.
Their phone number is 01895-833838. They'll send you a brochure and all you need to know for your trial flight.
Hope you enjoy it!
MS