Its surprising to hear GAD won't touch an SL55, but that is their loss.
I would thoroughly recommend MSL Performance who have a branch in Essex
The best remappers of AMG's in the UK IMHO and plenty of members here whose cars have been remapped at MSL.
The usual route for an M113K engine is an 83/77mm s/c pulley and remap taking you to 550 - 600bhp depending on which pulley you choose. He may suggest further cooling aids and some exhaust work to enhance that V8 rumble.
Prestige & MSL Performance
7 Coppen Road,
Dagenham,
Essex,
RM8 1HJ
020 8592 8849