However, are you sure that you have to use the 'P' plates? Is there a rule now that all new drivers have to have them, because I never bothered to use them after I passed my test at age 17, and neither did either of my younger brothers (the youngest passed in February this year). I've always been under the impression that they were optional.
"P" plates in Great Britain are still completely unofficial and optional. They've only been around for a few years, and in my opinion are a waste of time anyway (I figure that good drivers will be courteous anyway, and the arrogant, impatient driver can see a "P" plate and be tempted to take chances that he might not otherwise have taken).
Northern Ireland has an "R" plate which
is a legal requirement for newly licensed drivers, and imposes several restrictions for the first year, such as being limited to 45 mph (I'm not sure if/how that would apply to somebody already licensed elsewhere).
But as far as mainland Britain is concerned, it's only the "L" plates
* which are mandatory for learners, and you can throw those away the second you have your pass certificate from the examiner.
* Or alternative "D" plates in Wales.