That's quite strange. Sometimes for some reason my IP address also does this (it did it in the middle of watching Dr Who!) and it also tells me I need to be in the UK to watch the player, a simple refresh of the page usually fixes it for me though.

Other than that I'd have no idea why your ISP would assign a non-british IP address to you, if the above solution doesn't work I suppose you could always call the ISP and see what they have to say about it.

Although that reminds me recently that some ISPs have been thinking about blocking BBC iplayer because it apparently is costing them a lot of bandwidth!