The question is... Who did the brake pads on March 12th?
If they couldn't give you a "heads up" that the brake discs were on their last legs then it's time to find a garage/ mechanic who will... A brake disc will loose about 1mm of run thickness every 36 month on average usage and even a longer period on the rear so they had no excuse to say "it all of a sudden wore down"
It's common and good practice to inspect both axles for pad and disc wear even if changing pads on the front axle.
front disc is 22mm vented new to 20mm minimum when worn
rear disc is 10mm new to 9mm minimum when worn
So somebody hasn't helped you mate.
At least with these Kwikfit, Halfords and ATS centre's etc. You get a lifetime guarantee and they often supply a free brake pads and discs for life offer as long as they are periodically inspected by them... You don't have to take it back but if you aren't handy, then it's ideal... Just take any future faults with a pinch of salt and coincide your brake check just after an MOT, then you will know if it passes that, then the car suspension, braking and tyres are fundamentally sound..
Use and abuse their system