If you are more to the East of London, Essex way....
Seawing Flying Club @ London Southend Airport
(Learn to fly in Essex | Seawing Flying Club Southend) 01702 54 54 20
This is where I obtained my PPL and where I am currently a member and flying out of. Very friendly and relaxed club which is member owned, so you obtain a share in the club and its aircraft from becoming a member.... they do trial lessons in either a 2 or 4-seater and for either 30 mins or 1 hour.
Access by road via the A127 or by train from London Liverpool Street on the line to Southend Victoria directly to the new airport station.
North Weald Flying Group
(http://northwealdflyinggroup.co.uk/) 01277 225209
Another Essex-based friendly flying group operating out of North Weald Airfield and again there is a friendly and relaxed, yet still professional feeling to this club. Trial lessons can be arranged.
The airfield is 10 miles south of Stansted and can be seen and accessed from the M11 (A414, Harlow Junction). Take the A414 towards Chelmsford and then follow signs from the A414 and on entrance to the field, follow the perimeter track all the way around the field until you arrive at the distinctive Squadron building.