As beowulff says, there are pad wear indicators, a little tab on the brake pad that hits the rotor to cause noise before the wear gets so bad that it actually harms the brake rotor. The noise usually happens when turning at first. If it started when turning, you probably need new brake pads.
I just spent a month of Saturdays chasing down my own brake squeak after changing a rear axle and bearings. There are little metal clips that fit into the brake caliper bracket where the pads sit. Mine were loose and allowing the pads to shudder and create a noise like something hitting the caliper. A couple of dollars for new clips (I don’t know what they are called) and I got it. There is also a brake squeal compound available at auto parts stores that you can apply to the metal parts of the back of the pad that will help. Disc brake squeal is that common. I did both changing the old loose clips and the anti squeal compound and solved my problem.
Disclaimer: this advice pertains to disc brakes and rotors, not drum brakes. And if this is your MG, Johnny L.A. I can’t help you, maybe God can.