Right then, not 100% sure on the GB/D thing - i'm pretty sure the euro stars with any initials are allowed here, however, for a uk registered car, the font is illegal, the materials are wrong and they will not have the required kite marks, makers post code, etc that are now required by law.
No idea if they will pass a genuine MOT either!! Suppose it depends on the tester!
All the documentation is on the dvla website
http://www.dvla.gov.uk/vehicles/number_ ... marks.aspx
Having said all that, i've got them on mine, and illegally spaced - been spoken to by old bill 3 times now and they've never mentioned the plates or spacing!!
They're fixed on with velcro though and there's a proper set in the boot just in case!!