If you still cannot leave feedback after trying the option already provided in the first reply to your post, try logging off of eBay UK and log onto https://www.ebay.com, which is the USA version of the site. If the problem you are experiencing is due to a glitch specifically affecting the UK version of the site then by logging into the USA eBay website (or any other eBay website that you care to use), you could potentially end up managing to get around the problem by trying to leave feedback in the manner described here.