I would say that a good relationship with your MOT tester is more important than price.
Although I am classed as Motor Trade, I never ask for a discount and pay £40 for my tests. I also go to a testing centre that is very strict, but due to the mileage we cover, I am happy to know that the car is in tip top condition. However, what makes the big difference to me is that my MOT guy will allow me to accompany him when doing the test, and he will point out to me various unrelated mot aspects that may or may not need attention at some point soon, and will allow me to rectify anything if needed. He is not afraid to fail something in front of me either.
For that, I am happy to pay the price - its quite a small one given a years ticket and a second set of eyes