It's weird. I visited the site on my phone and it had the dollars in US, and I tried to check out and got the same error about them not taking my currency. I got on my laptop and the currency showed up on GBP. It picked USA and checked out (I logged in both times). Off I went to paypal where it told me the exchange rate and let me purchase it.
I don't know how to make the website show up in GBP, but like I said, I was logged in both times.
Try clearing out your cookies, temp internet files and all that.
EDIT: I think the key is making sure underworld's site shows the prices in pounds instead of whatever your dumb non USA currency is,
