4chan is and has for a long time been in cooperation with US law enforcement. You can literally, right now, enter the site and when reporting a post there's an option to report a post as breaking US law.
If the UK is not happy about how the site deals with such matters, the UK can block the site.
>Next on the escalation ladder is govt writing the rules for you that you either take or leave
No. This is not "next". This is "now" but the UK doesn't want to actually look to be doing what they are actively doing, and now we've got this mess.