Sounds a complicated nightmare and why drive around on the wrong side of the car for the next X years! I bet the warranty would be US as in no warranty

or it would revert to UK warranty which is rubbish compared to the US one! You need to talk to Honda UK about this in writing!
We never got the 10 year 150,000 mile upgrade in UK

in fact UK Warranty it's only 80,000 miles and 8 years, and only for those with a 2003 or younger genuine UK car, unless you had an extended hondacare warranty on a pre 2003 car
Better to buy a UK car or one that has been already imported IMO, there are a few about on Autotrader and Honda UK sites if you look from time to time. Up to you of course. I also think a left hooker would be worth diddly squat in UK resale. I might be wrong though.