A number plate was originally issued by DVLA on a totally random basis - the fact that you now own (through procurement or otherwise) a number plate which looks like a trade mark is not an issue. You are not using it to directly advertise any competing product when it is atached to a particular car / bike then you should be ok. Audi may be have a case if you put AUD 1 on a Fiesta (sorry guys if any of you own one!!) but do you really think that it would be classed as passing off of a brand?