Brakes probably are not faulty, just worn.
How loud is the squeak - metal/metal loud or just annoying? Maybe the pad backs have fallen off.
Only way to cure it is to change the brake pads and sensors and possibly the discs. I would start by removing the pads/calipers, giving them a clean and check nothing is missing and refit them.
The sensor is in constant contact with the brake PAD but doesn't squeak as its plastic.
What did the dealer actually do for £90?