Which to be clear, is nothing but a fiction you built yourself in order to insist on your failed arguments, 4chan is a 18+ site and it enforces the rule. It's literally the second rule of the site just below not posting illegal content.
Here, 4chan's design works against answering the question satisfactorily; their commitment to anonymity makes it difficult to say whether they really keep minors off the site (purely anecdotally, the answer seems to be "Not really; everyone knows click-through confirmation is a joke," but that's anecdote). OFCOM allegation, indeed, is more-or-less that 4chan pays lip-service to keeping minors out but fails in practice, and they demand better of service providers then "Well, we told them they're not allowed and if they make it too obvious to ignore we give 'em the boot; what more could we possibly do?" Because OFCOM has a list of what more they could possibly do.
There is a nonzero possibility that the Internet as a whole - including fora like 4chan - has become deeply enough integrated into daily human experience that 4chan having an "Are you over 18" clickthrough is no more acceptable than a 21-and-up club leaving the back door unlocked and never carding once you're inside.