http://www.legislation.gov.uk/ukpga/2017/30/contents/enacted

The idea was, Porn sites need to obey UK government rules, if they /don't/ (and why would, for example, a Canadian porn site obey those rules?) then a censor (the BBFC that took on this role for video got that job) directs that ISPs should block the offending site using DNS blocks.

In practice it keeps getting pushed out because it's hopeless from all angles.