UK not a shipping destination option by International Shipping program? [buyer]

Two listings I'm very interested in from US sellers, one of whom has confirmed they're in the International Shipping program (the other hasn't responded), don't have the United Kingdom included in the long list of shipping destinations but also don't have it in the explicitly excluded list. Instead of a postage price I just see "May not post to United Kingdom. Read item description or contact seller for postage options."

The seller I'm in contact with has no idea why that's the case, as they haven't set the UK to be excluded. They also don't seem to know how to fix it. Weirdly I swear I saw that listing provide a shipping price to the UK earlier this week, but when I went to checkout yesterday it was rejected as an invalid destination, making me wonder if this something ebay itself is doing.

I'm also pretty sure I've bought from other US-based International Shipping program sellers recently without issue. Also, on the list of countries that are available are places like Ireland and France, everything around the UK but not the UK itself - these definitely aren't "NA-only shipping" listings, they ship internationally but just not to the UK.

1. Is this something people here have experienced before, and might know what's going on?
2. Is this fixable on the seller's end, and if so can you tell me what to tell them to get it fixed? I've never sold on ebay, I have no idea what it looks like from a seller's perspective so I need your help on this.