An MOT is a basic test and checks whether your vehicle meets the minimum legal requirements for the items required to be tested under section 45 of the Road Traffic Act 1988.
It does not tell you how long your vehicle will continue to meet these minimum requirements. Therefore, you may still be prosecuted for driving an unroad-worthy vehicle before the end of the current test certificate. You should continue to maintain your vehicle to ensure that it will always pass an MOT.
Brakes are not checked for condition, only an efficiency test is carried out. If alloy wheels are fitted and the tester can see the brake pads through the wheels and the pads are below 1.5mm then this shall be reason for a fail because they are too low, but if they can not be seen through the wheel then a 0.5mm pad will still pass the MOT, even though that it is getting dangerously close to running out of brake pad material. Brake discs will fail if any signs of cracks or covered in oil or brake fluid.
Remember the wheels are not removed for the MOT test.
Just because your vehicle passes the MOT it does not mean that your brakes are in good condition.
A service is a more in depth check, its not a simple changing of the oil and filters etc, any advisories that would be mentioned with your MOT will be rectified, ensuring that your vehicle meets a high standard, to ensure you will stay safe and legal rather than only the minimum legal requirements being met. The wheels will be removed, checking the brake discs and pads thoroughly and on some rear brakes the brake drum and shoes cleaned, inspected and adjusted.
Below is an image of a brake pad which had just passed the MOT, luckily the vehicle was in for a service and MOT. As you can see there is very little pad material left, the edges of the pad had been scraping on the rusty lip of the brake disc leaving no material left, actually scraping down to the shiny metal of the brake pad backing.
Even though the pad material was extreemly low, there was still just enough touching the disc to give an acceptable efficiency reading, allowing it to pass.
This shows how important it is to get your vehicle serviced regularly, not necessarily at a main dealer with their very high service costs but at a good reputable garage, they will give you good honest advice and great service for a lesser cost.