That assumes UK deploys technical measures to prevent their own citizens from accessing the website, which costs more political capital than fining a corporation. Or makes it illegal to access the site, which is even more unpopular.
Realistically, UK is a big market for 4chan, but is 4chan big enough for UK? What share of its 70M+ population will flip their vote because of this specific case? How many people will just switch to Reddit or something else and won’t even connect that block to any political party?
The difference is significant.