Robin, I would almost have to agree, the Imprted orp is just a fluffier Orpington making them appear larger. Most of the Imported orps out there have been crossed back over US based orps anyway to attempt to get closer to the standard of the Orp here in the US.
I would be very carefull if you purchase any "Imported" Orpingtons anyway as it is likely not what you are really getting unless they have the import paperwork and can document the the importation process they are likely just bumping prices for the new fad.. The folks that will spend teh time and money to import things seldom let much go until they have raised them out and choose for thier own breeding programs since it averages 4-5K a bird between bird, shipping, broker fees, and quarantine period at $20 a bird per day for 30 days.
Here are the reg. on this side. The part where it says "some hatching eggs will have to quarentined for 30 days" just slays me. LOL
http://www.aphis.usda.gov/import_export/animals/poultry_eggs.shtml
Here they are for the far side of the pond.
http://www.defra.gov.uk/animal-trade/exports-non-eu/
Basically the UK will follow the rules of the importing country.
And here is the history of the Orpingtons.
http://en.wikipedia.org/wiki/Orpington_(chicken)