I have done a considerable part of my flying to / from airfields under the London TMA.
If you come from the north, talk to Farnborough North on 132.800
If you come from the South-East, talk to Farnborough East on 123.225
If you come from the West, talk to Farnborough West on 125.250
If there is nobody there - talk to London Information on 124.600 (if memory is right)
All of which will be happy to provide you with a Basic Service (or if Farnborough, they will happily pass on Traffic and Deconfliction services providing controller workload). They will pass on the local QNH on first contact with the ATC unit.
Bear in mind the accuracy of the altimeter vs accuracy of transponder reported altitude - as a general rule I make sure to be at least 100ft below the TMA, I often fly at 2,200ft to avoid the possibility of accidental zone incursion. There are a few "corridors" which are a little tight, and you need to keep a good lookout at all times. ie: between Luton & Standsted where there are a few airfields in between, and relatively stringent crossing requirements and between Heathrow and Gatwick where you have people from Redhill, Kenley and Biggin. It has never been an issue for me, but just "keep your eyes peeled, and your ears flapping"!
If you need any more help with your flight planning - I'd be happy to help!
Good Luck For Your Flight!
Alex