You can buy emergency filling stuff from Tesco.

Also yes, you can make an appointment at your local 'emergency dentist'....which is what I did a couple of weeks ago when one of my fillings dropped out.

I also had a post crown, but at the side of my mouth, and yes it fell out....a few years ago. It is in a little plastic bag in with my other 'bits' now....and a nice gap in its place. One day....