I am interested too. I think the one in Bengeo is meant to be good but when I drove past the queue was making it really difficult to drive along the road as you couldn't see far enough to overtake them (parked cars adding to the visibility issue).
I don't know the name of it..but there's one in Mimram Road, Hertford. A short distance after you turn into the road from the A414 it's on the right. Around £15 to wash and dry the exterior and clean inside thoroughly too. Lovely guys and the car's always sparkling afterwards.