You could try doing what I've done in the past whenever I've experienced problems with making payment.
First of all, ensure that the item you're trying to pay for is saved in your Watch List. Log out of eBay UK and log onto www.ebay.com, which is the American version of the site. Go straight back to your Watch List and locate the item in question, then try to pay for it again. If the problem you are experiencing with making payment is a glitch that is currently affecting the UK version of the site then by logging onto another eBay site that has not been affected in the same way you should find that the payment will go through successfully.
