Pressure settings under a TMA
Apologies for what is I'm sure a simple question, but I'm a simple soul...
BLUF - what pressure setting do I need to use when flying under the London TMA? I'm assuming the London Regional QNH?
Background - I'm a low hours PPL living in carrot cruncher land, where the skies are open, the vistas superb and there are always a plethora of suitable looking fields to force land in if the donkey were to stop making the normal noises. BUT, I want to challenge myself, want to fly further and do more things. Some mates of the wife live in Southend, so I've done a bit of flight planning and reckon that a visit from here to there is perfectly possible, so long as I can sneak under the London TMA. But that puts me below the transition altitude and therefore flying on a local QNH. The last thing I want to do* is infringe some airspace and incur the inevitable wrath of the headmaster, which then raised the question of how is the base of the TMA calculated.
*Not strictly true - I think I'm even more allergic to crashing.