To be honest I can't say that I have the answer to your question as I gave up selling on eBay when they got rid of PayPal and introduced eBay Managed Payments, and a lot has changed in that time.
Having said that, however, have you tried logging off of eBay UK and logging onto one of eBay's overseas websites to see whether the buyer's User ID will show up on your Sold Items page there? If not it might be worth giving it a shot, especially if the absence of the buyer's User ID on your Sold Items page is the result of a glitch or a new eBay "improvement" that is specific to eBay UK. If you try that and you can see the buyer's User ID on your Sold Items page then it may be worth logging onto one of eBay's overseas sites as a workaround option in order to get the User IDs of your buyers until such time as eBay rectify the problem on the UK site.
