seat OA
The reference is at
www.ais.org.uk
You need to register (free for change!)
AIC 36(P170)/98 entitled 'Fuel reserves for aircraft approaching the UK'
It's a .pdf file that I can't cut and paste from.
In essence it re-iterates the JAR fuel policy minima but goes to say that the London TMA is a bit of a special case because the 'no delay expected' of 20 mins quite often applies.
It goes on to point out that a diversion accross the TMA will probably not be a straight line.
The clear recommendation is not to arrive in the London TMA with the JAR legal minimum fuel - ie: have that 20 mins 'in the bank (tank?)
Have a read and make your own judgement.
Rgds