I’ve never worn a thong on a UK beach but have in other European countries. You’re right, I think we are a bit uptight when it comes to nudity and, I’m sure that includes male thonging, in comparison to mainland Europe.
I think if you were on or close to an official naturist beach then legally, I’d assume you’d be ok. Plus, you’re obviously not fully naked so I can’t see how this could ever be an issue?
Given the situation we are sadly in with cancelled holidays and travel restrictions, I’m hoping to visit some UK beaches later in the year. If I get that opportunity, I’m definitely going to thong for the first time on a UK beach.