Used SxS in the UK are a drug-on-the-market ... you have to kick them out of the way to get firm-footing on pavement.
Where, geographically, are you going?
What are you wanting (as in: maker, quality, gauge/chambers ..)?
Do you know how to fully evaluate a used gun, or do you need to buy from a dealer who has gone through it, and tuned/tweaked, etc.?
Have you looked at the BATF site to see what's involved in importing a gun? There's some grief, but it's not a real big deal.
On google, you can find numbers of Brit arms dealers, and we have more than a few Brits here who may be able to assist.
Bob