IncendiaryLemon
Captain Karen
If you buy from Amazon.com (you can use your Amazon.co.uk account there too, and they'll charge you in GBP so no transaction fee from your bank), you can put the import fees down in advance. So if you get charged them, Amazon will use that money and you won't have to pay the handling fee @IncendiaryLemon mentioned. And if you don't charged, Amazon will refund you the money you put down.
Perhaps a better option might be to buy from a UK reseller, such as United Publications (aka UP, UP1, UP1 USA) and I think Otaku.co.uk do it. Here they import the item and then send it to you from within the UK, so they deal with all the tax and handling fee and such nonsense, and you just pay a single fee and receive the item. I will however, this is legally questioned, so you might not be as protected as if you were buying a UK item from HMV or wherever.
If you're talking UK reselling, eBay and Amazon UK Marketplace. I know I got my Bakemonogatari AoA BD from eBay and I got a hell of a good price on it vs UP1 and the like.