The base of the London TMA is defined by the London QNH (not the regional QNH). The transition altitude within the extent of the London TMA is 6000 ft, so you should definitely fly on the London QNH when below it.
Sorry, I know I should know this, but how do I find the transition altitude when it differs from the usual 3000ft? The fact that the TMA base is stated as QNH is obviously a giveaway, but is there anything else? (I don't have a map in front of me so apologies is this is a dumb question.)
Cheers, David.