Except in this case, Imgur does not have a legal entity falling under the UK's jurisdiction. They are purely a US based company. It's not like Google, Apple, etc. that have offices in the UK.
This particular fine is the UK trying to extend its jurisdiction to entities that it has no sovereign authority over.
Just because some UK user might visit my website doesn't mean I now have to follow all UK laws if I don't actually do business there, and don't intend to.
Blocking the traffic is how we end up with the balkanization of the internet.