what pressure setting do I need to use when flying under the London TMA?
When flying under controlled airspace you use the QNH of any airfield under the controlled airspace. The regional pressure setting is designed to guarantee you won't hit the ground; it can't also guarantee not to put you too high, so you can bust airspace following it.
If you haven't already got a QNH of a suitable airport you can pick one up from an ATIS, or you can ask a controller. In the case of London you can ask for "London QNH" without worrying about particular airfields - they'll give you one that can be used to avoid airspace busts.