If Imgur never had UK presence, then yeah there would be no teeth. But if you're doing business in a country you can't break the law then leave and expect them to just ignore what you did during that time.
How do you expect the "pull out" to happen? They must have had a UK bank account or similar, whose transfers won't get approved as they're trying to escape from criminal prosecution. Or they'll work with the US to ensure responsible individuals are held responsible.
It isn't exactly the first time someone/something commits crime in a country then try to escape, there is lots of ways to work with others on this.
If you're the US you call planes of out the sky that have representatives and owners of the companies on them.
I'm assuming any leadership of Imgur would want to avoid going to the UK for the rest of eternity.
I heard here recently during a similar discussion (about 4chan and this same British watchdog agency) that the US does not allow extradition of its citizens for breaking non-US laws if the behavior is legal in the US.
How so? None is needed to take revenue from UK-seen ads.
I would do that (after appealing) and be done with it.
American companies are too use to being able to bully their way in America. Some countries do have better consumer protection laws.
May US voters put America First over international law.