The UK red tape is nothing compared to the US! The UK allows grey market stuff, the US doesn't, as a general rule. (Thanks to the lobbies of motor vehicle manufacturers.) To make a long story short, if you want to license a non-US market bike for the street, you will have a BIG headache on your hands... unless you know someone who can pull the wool over the eyes of US Customs, the EPA, DOT - or can satisfy their myriad of requirements. Then there's state DMV. (Bill Gates gave up on trying to make a Porsche 959 comply and just brought it in as a track toy.)
OTOH, if it's for non-road use - like a track bike - it's usually pretty easy. That would be one way to curcumvent the feds. You'd just have to convince your DMV to register the imported bike with no federal docs. Might be easier to do in a non-border state where they rarely deal with something like that. that's usually how unobtainium bikes get into the US. That, or they're brought in by a foreign national and sold.