Stapleford Flight Centre - north-east from Central London
I trained at Stapleford for my PPL and now I'm in the middle of the IMC rating (a.k.a. IR(R)) course. The Stapleford Aerodrome is located near the M11/M25 junction, can be reached by car from both motorways and is a short taxi ride away from the Theydon Bois station on the Central line. The prices have gone up over the years but I still think they are relatively cheap, without a detrimental effect on quality of training or the fleet. I quite enjoyed my training there.
Stapleford seem to be focused on commercial students as they also do CPL/ME/IR + help in type ratings and sometimes take less notice of us leisure flyers but if you plan to go commercial, that should only work in your favour!
You seem to want to get it right first time when you go for a trial lesson - can I suggest that if it's a career choice, then 2-3 trial lessons in different places might be in order, as a good way to get a feel of each place before you start your training. Just get a logbook for your first trial lesson, so that you can log all of them if you decide to sample more schools on that basis!
All the best,
/h88