I bought an extinguisher, and it has all the european/british standard/kite marks and stuff, but doesn't have a certificate to say its been inspected.

Chill, it will do. just stick in the van.

pull the pin and fire it off at the missus .. if it covers her in white smoke its good

You can imagine the officialdom scenario, you can't use that extinguisher on that fire mate, it aint got a cert on it. "I don't care if it does work you can't use it". :D