There is a place near me in Crayford called Just MOTs where a few friends of mine have gone before. £35 and because they aren't a fully fitted garage, they wont try and generate work because they don't have the capacity to do it. They'll jsut do the little odds and sods that might need doing like wipers and blown bulbs. Only downside is if the cars fails you will have to find somewhere else to get the work done, but it's a good punt if you're confident the car will pass.
Edit: Apparently there's quite a few branches of this place....
http://www.justmots.eu/