What about car's with steel wheels do they not have to take them off so they can see the front brakes discs?
It's a test requirement that no part of the car is altered or replaced during the test, including the wheels. If they did, you have a right to challenge the MOT result.
You should find that all cars, steel wheels or not, have some facility to visually check the brakes. As MikeO said, you can see an awful lot of the car when it's up in the air and you're underneath.
Then again, the MOT is a complete waste of time anyway. There is no way I'd consider an MOT pass as being a mark of being roadworthy.
Sorry, I got a bit off-topic there...