Do they run their own ad network, or do the ad networks take the money from advertisers and cut Imgur a check? Maybe instead of trying to enforce your standards on every little site on the internet, you should just focus on the people who actually have a direct point of contact with money coming from UK businesses. (Yes, the ad networks.)
It’s completely absurd to say that some hobbyist would have nexus in the UK because they run a Google Adwords campaign to get some occasional pocket change from their project. Pre-Internet, it would be like going after a US magazine because someone brought home a copy from the US. Websites are not global entities by default, somehow responsible for obeying laws across nearly 200 national jurisdictions and many more state/provincial/local jurisdictions, across different languages and legal customs. Completely absurd! Who do you think you are to demand such a thing?
On the other hand, I think it would be perfectly fine to say that UK domiciled ad networks cannot put their ads on sites that violate some arbitrary standard. (An anti-freedom law to be sure, but at least it’s consistent with common international conventions.) This puts the onus on the ad network, rather than the site owner, who may not know or care who is visiting or from which country.