Given that you said it was a UK seller I'd say there are two possibilities that come to mind. The first is that the seller only ships to certain regions of the UK, so if you live in an area that the seller has put a block on then you will not be able to purchase the item. If, however, it's due to a glitch affecting eBay UK then there is another option that you could try which may result in you being able to successfully pay for the item.
Although it's not possible to say that the problem is definitely being caused by a site glitch, for the time being treat it as though it is, as if it is indeed down to a site glitch then the following workaround option may well provide the solution, particularly if it is a glitch that is specific to eBay UK.
First of all log off of eBay UK and log onto one of eBay's other websites. Whenever I've experienced problems with site glitches in the past - including glitches which have prevented me from making payment - I've logged off of eBay UK and logged onto www.ebay.com, which is the American version of the site, which I've always found to be very effective in getting around eBay UK specific site glitches. If you log onto one of eBay's overseas websites try to make payment that way. If the payment goes through on the overseas eBay website and the item shows up in your Purchase History with a black £ icon, representing successful payment having been made, then log off of the overseas eBay website. Log back into eBay UK and check your purchase history to see whether the purchase is showing up in your Purchase History on the UK site. If the problem with payment was indeed due to an eBay UK glitch affecting the checkout then the item you were attempting to buy ought to now be showing up as having been paid for.
One final note that you may need to be aware of - if the seller has been switched over to eBay Managed Payments then you may see an hourglass figure for a while before the £ icon goes black and confirms that payment has been made. I recently purchased an item from a seller who had been switched to eBay Managed Payments, and the hourglass figure stayed like that for a full twenty-four hours before updating and showing the black £ icon to confirm that payment had been received by the seller! Therefore, if the problem isn't due to a site glitch then it may be due to teething problems with eBay's new Managed Payments system, although if this is the case then the hourglass figure ought to update eventually. If you do believe the Managed Payments to be the culprit then keep a close eye on your Purchase History, just to make sure that the payment clears.
In the event that on checking your Purchase History regularly the signs appear to point towards this being a Managed Payments problem then it would probably be worth your while to send the seller an e-mail via eBay Messages to let him/her know that you have paid for the item but the payment is showing up at your end as though it's taking time to clear. Should you decide to do this then it may well help to reassure the seller that your problem is genuine and that you are not a timewaster. A person with no intention of paying for the item would more than likely deliberately avoid entering into any correspondence with the seller regarding payment so this ought to reassure the seller, especially if he/she has only just been switched over to Managed Payments and didn't have any payment-related problems whilst he/she was receiving payments via the old system.
