Have you logged into www.ebay.com recently, which is the American version of the site? If so then that may well be the reason why the currency is showing up in US Dollars. Try logging out of eBay, then clear all caches and cookies on your computer and log back in again using www.ebay.co.uk.
If you think that you may have inadvertently logged onto the USA version of eBay by typing in the .com address instead of the .co.uk address then add the eBay UK site as a shortcut option to your browser so that once you open the browser you can just click on the eBay icon when you want to log in so that it takes you straight back to the login page of eBay UK without you having to type the entire website address in manually on the search bar.
