I don't know what shocks me more,
£180 for new rear pads fitted or not double checking the cradle bolts are tight.
Presuming the pads were stupidly expensive at £50 and the garage charged 30 mins labour. That would equate to £260 per hour inc vat labour charge.
Nice work if you can get it.
For sure mistakes happen in any environment, with what you describe has happened there will have been a warning noise before the end result.
I have taught my wife and daughter to listen to there cars for any odd sounds as one day stopping and checking may save their lives.
If I was you approach the garage in a polite manner, ask for any money you are out of pocket for regarding recovery and a goodwill gesture for the poor quality of workmanship.
Threatening this and that will just get you nowhere I suspect.