There is a little more to this...
Correct, the transition level is the first available level which, on a given day, is physically above the transition altitude, and that will depend on the pressure. The Minimum Stack level is the lowest level above the T/A which gives 1000' separtion from it.
The reason why the London area has a different transition altitude to the rest of the country is (was) the need for procedural separation of traffic within the London TMA, as the amount of traffic from the various airports increased.
Take Brookmans park as an example: With each of the then 4 airports routing traffic through BPK, a 3000' T/A was no good because this left no room underneath for anybody else in what was still a significantly 'procedural' environment. By using 6000', Stansted's can go through at 3000', Luton's 4000', Gatwick's 5000' and Heathrow's 6000'. Thus, they were all procedurally separated and on the same setting which made it easier to release traffic off the various airports. Of course these days everything has changed and we are now reliant on radar separation to the very largest degree but even now this is still the basis for the separation of SIDS.
Point 4 <img src="smile.gif" border="0">
[ 16 February 2002: Message edited by: 120.4 ]</p>