It was a really clever move on Imgur's part. Their blocking the UK has nothing to do with the Online Safety Act: it's a response to potential prosecution under the Data Protection Act, for Imgur's (alleged) unlawful use of children's personal data. By blocking the UK and not clearly stating why, people assume they're taking a principled stand about a different issue entirely, so what should be a scandal is transmuted into positive press.

It blocks many more countries than just the UK because it's the lowest effort way of fighting "AI" scrapers.

imgur was created as a sort of protest against how terrible most image hosting platforms were back then, went down the drain several years later, and it's now just like they were.