Well, it's not as simple as that and i read through a lot of Ofcom documents and discussions, to get the sense of it. Note that i still sensed there is some leeway in there and hence we will likely be just fine on short to mid term. Porn sites and similar is likely what will get some scrutiny. The way i understand it an authority will still reach out before starting procedures and before issuing possible fines.
But lets say the most draconian implementation would to happen, over the span of next couple of years. Of course then site owners would be responsible, not only to verify age but as this is public forum the content that is behind age verification wall wouldn't be accessible to public internet any more. So basically, yes, on public internet one would need to act like lets say 10 year old child, on age verified section of the forum one could i guess still be allowed to fat shame a bit, or talk about pro cyclists diet, or even to encourage children to ride a bike. As having such diet or riding a bike often results in injuries and eating disorders hence such "self-harm" suggestions would need to go under age verified section.
As for the solutions i remember the first time i joined one of the biggest Slovenian forum community, when sending a PM it said this message will be visible to you, participant you have send it to and to the moderator. Now that IMHO is a solution too and doing it like that you are IMHO in compliance with the said law in the context of cycling orientated discussion forum, targeting adults.