How is WordPress redirecting/rewriting my URL?

Try the wordpress maxmind plug-in. I’ve never used it myself, but it should do the heavy lifting of identifying a user’s location by IP address.

You’ll have to add some functions to dictate what to do with that data in terms of redirecting visitors (the plug-in only figures out where they are). You’ll probably also need to add bits to handle situations when the location isn’t recognized (or isn’t in the UK, for example).