Major airports are expensive places to do anything - it's not very surprising that nobody is organising instructors seminars at them.
The AOPA courses at High Wycombe would be about 35 minutes by taxi or hire car from LHR. I went on the last one and quite enjoyed it, although very "old school".
Wellesbourne airfield (On-Track) is about the same from Birmingham airport. I've not done one of their courses, but have heard nothing but praise for them.
G