You reminded me of Bridgemoto immediately.
Bridge Moto
They brought in a container load of USA sourced bikes a while ago; all brand new - DR650s and KLR650s (sorry, off topic!!) from memory.
Last time I looked at their website they all seemed to be sold; the dealer is reputed to be very helpful so give them a ring.
The asking price was around £5200 IIRC.
They also import second-owner machines from overseas and they are not the only ones to do this.

I talked with a vehicle dealer a short time ago who said that most vehicles for sale in the UK are advertised on either ebay, gumtree or autotrader; he concluded that between those 3 the market is more or less covered as far as UK advertising is concerned.
For sure, if you want to visit the north Americas, purchase in the USA, ride there and then import to the UK it should be economic overall; you wouldn't be the first to do this.
Alternatively, look toward the European market, say France or Germany; denizens of those countries have always tended to like the dual sport type of bike compared with the UK infatuation with race track replicas, so there tends to be more of them around for sale second hand although a new DR650 is not for sale, formally, anywhere in the EU because of the pollution regulations.
(It can still be done under single vehicle approval regulations for the UK, at a price of course - see Trip Paperwork for that discussion).

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Dave