Just had my bike tested. The information in that link is no longer valid. Any bike registered after 2001 must have a standard size plate, with no border or fancy font for the lettering. It must also bear the name and the postcode of the plate manufacturer and the British Standard code for number plates. It can, however, have the blue panel for GB or EURO down the left hand side.
Had a long chat with the tester who said that there are more things to fail the bike on, regarding the number plate, than the rest of the bike. Apparently down the automatic number plate recognition cameras now!!