Maybe go back to the repairer quickly to see if they can salvage the old VIN plate, if it's not too badly damaged. You could then have it riveted back in the proper location.
I believe as long as you have at least one VIN identification that is complete and legible you can still obtain an MOT (drivers door pillar and one on the dashboard passenger's side).
If you want a VIN plate on the crossmember have a look here:
http://www.vin-plate.co.uk
They don't provide the plates stamped, but you can purchase a blank Vauxhall VIN plate and buy a punch kit to do it yourself.