There are still un-stamped cases around, but correct font stamps are very rare. It's not illegal to re-stamp cases, only to use cases you know to be stolen.
Even if they've been stolen, you are not breaking a UK law if they've been "recovered", i.e. if the bike or parts have been recovered and numbers removed from the police stolen property lists. The engine number may be anything you like, I know of a bike in Croydon where the engine number is "80LL0...", it's perfectly legal, and it's on the registration document. Mind you, it did take a while to convince Croydon police, when we got pulled over!
The last places I know where un-stamped cases were on the shelf, was Millers in Hastings, and Ken Chaters, in Bexley. Both now sadly closed, but cases do turn up at auto-jumbles.