I think it really does depend on the MOT tester /station. Some of them are just there to rob you and all your money. I recently had a Toyota yaris taken to a city council testing station and it passed despite having a lot of things wrong with it (like a hole in the cat that was bodged up with sealant). It passed with just one advisory.
I took my zed to an independent garage and they failed it for things like "rear valve stem seriously damaged" - I never lost a noticeable amount of air on either of the rear tyres (of which supposedly the valve stems were damaged). They charged £18 a tyre to replace the valve stem... Some of the stuff on the fail sheet is just ridiculous...
Edit: I had the rear tyres later checked at another mot testing station and they're absolutely fine with about 3mm tread left at the time.