Is it against the law to wear a diamond wedding band working with food in the pub catering job as my boss wont allow me to wear it?

It might not be against the law, although you should check your local ordinances. It is against safe food practices to wear any jewelry - particularly those with stones. The stones could present a possible foreign object hazard if any came loose and ended up in the food. The nooks and crannies around the stones would be difficult to keep clean and sanitized, and could present a biological hazard.

If you really can't take it off, talk to your boss about the possibility of wearing latex or vinyl gloves.