Still not UK, but I've been to the physical site of www.themedievalshop.com (funny way of putting it) - they do do katanas - nice ones, despite their apparent absence from the site. But while I was there looking at katanas I did a bit of browsing and picked up a claymore - "oh, yes I like this" - then put it down before people nearby came under threat.
Talking with the storekeeper; it seems local law is a bit hazy about carrying swords. You're not going to be able to carry them with you when you're going down to the shops, but you are allowed to go out and use them (with other consenting adults) and you have to go somewhere to stage battles and you can't exactly leave your swords in the park when you go home....
Be warned, though, that you'll be looking at a price tag of at least £150 or so (assuming Customs will even allow you to import weapons in the first place). I somehow suspect that other sources might demand similar prices.
