Was out riding a lot this past weekend, and noticed how weak my back brake is

I can put all my weight on the pedal and the wheel won't lock. If i'm standing at a traffic light i can put a fair bit of force on it and still roll the bike backwards or forwards

The rear disc is badly worn and was an advisory on the last MOT, besides that, could there be another problem and in general how sharp are NC30 rear brakes?
Cheers all!
