This really only matters if you live in the colour-filled parts of the map, ignore otherwise

OFCOM is the Office of Communications, the UK's official communications regulator (Similar to the FCC for America, BEREC for EU, Roskomnadzor for Russia). It oversees TV, radio, telecoms, and parts of the internet. Under new UK laws, particularly the Online Safety Act 2023, OFCOM was given expanded powers to regulate online platforms.

The tl;dr is that 4chan has until 25 July to meet compliance in several fields (https://www.ofcom.org.uk/online-safety/illegal-and-harmful-content/investigation-into-4chan-and-its-compliance-with-duties-to-protect-its-users-from-illegal-content):

Remove illegal content (e.g., terrorism, CSAM, hate speech).
Implement age verification for certain content.
Have robust reporting and moderation tools.

The most important point is the age verification for 18+ content, which is likely not going to be done based upon what was seen on the technical capabilities during April.
What will happen is either:

All red boards will be blocked from viewing for citizens of the UK and will require ID for verification. (sets out a non-exhaustive list of methods that we consider are capable of being highly effective. They include: open banking, photo ID matching, facial age estimation, mobile network operator age checks, credit card checks, digital identity services and email-based age estimation; stipulates that pornographic content must not be visible to users before, or during, the process of completing an age check. Nor should services host or permit content that directs or encourages users to attempt to circumvent an age assurance process; 99% accuracy is required https://www.ofcom.org.uk/online-safety/protecting-children/age-checks-to-protect-children-online)

All ISPs in the UK will block 4chan.

Enforcement of this law only started this year, so nobody is quite sure as to what will happen.