Lol, I just paid £20 for my MOT today 
Go to a council garage, they're less inclined to sell you stuff.
I disagree 100% with that, as, dvla set a price, say £54, garages then have to pay a certian amount to dvla for how many ever mot slots they want..( i dont know how much it is, but am sure £20 dont cover theres...)
I reckon for certain places, they do it on purpose, do it cheaper TO find things wrong, to make thier monies up. Plus, theyre maybe not as thorough as a main dealer for example.. as we had a car in not long ago that had an mot done at a garage garage and failed on an item, so they brought it to us for work, we did that item, but also did a quick check of car ourselves (duty of care) and found all sorts wrong with it.. the vehicle was no way road legal..