They send an "enforcement" warning for him to comply, giving him another deadline.
When he doesn't meet that deadline, they'll open an "investigation" into the website as he hasn't complied.
After several months, they'll reach a judgement and pursue action (or not, drawing their feet forever - see 4chan). At first it'd be a "fine" (one they'd have to pursue privately since they don't actually have state-backing - good fucking luck) then another demand to comply with the threat of the fine emphasising the need to comply, accompanied by a deadline.
1. TV licence route: Null doesn't comply, then they go back to step one, apologising for the tone and requesting compliance by another deadline. Null doesn't meet it, they threaten to enforce the fine by another extended deadline, repeat ad infinitum.
2. Impotency route: Null doesn't comply, the threat of fine is out there, but they somehow have to enforce that fine via private channels. They may then approach the government with their findings, at which point a private court must determine whether their findings prove the site is in violation of the Online Safety Act and consequently ban the site or - funniest outcome - not. (If Null gets fewer than 7 million active users from the UK a month the site was meant to be ignored anyway)