You can get all the GT pack stuff on a JDM car too though... how about when you pull the sun visors down, do they have jap writing on them or Japanese?
Does the VIN on the chasis match the one in the windscreen? Do these match your V5? What colour does your V5 say? And is the car GM all over (i.e. engine bay, in the boot etc)?
As for the plate, i think the first UK cars were on a 53 plate, not 03, so thats a full 1 year older plate thats been put on the car. If it has been exported and reimported, it was a crap decision to put an old plate on it by the previous owner.
Hope you find some answers mate.