I wouldn't be so sure of that - the OSA appears to require sites to proactively prevent such content being posted, not just respond to user reports of potentially-illegal content. And, if you make a determination that something's fine when somebody in Ofcom decides differently - eg when a disgruntled ex-member inevitably reports it - the fine is 10% of global revenue or £18 million, whichever is greater. There's also the age-gating requirement, which isn't 100% clear at the moment, but will likely require the use of identity-verification services.
Worse than that, you can actually be held in violation of the OSA for over-moderating too - you have to have an appeals process, and if too many appeals are successful you can also be fined because this is such an ass-backwards law.
The law doesn't really provide definitions; those are yet to be published by Ofcom, but can also be changed by Ofcom at any point without any involvement from Parliament, so you have to keep an eye on Ofcom constantly. Oh, and there's fees too - which aren't yet published, but they're to cover the cost of enforcement (as if the fines don't do that already), and aren't necessarily related to revenue.
EDIT: No, apparently the fees will just be for the largest sites, so at least that's something.
Incidentally, it doesn't just apply to current content; it applies to all historical content too, so you're going to need to find a way to scan every post on the site and document the results.
The kicker, after all of this, is that it includes private messages too. Proactive moderation of PMs means, basically, that they're no longer private.
It's vastly more onerous than just knocking up risk assessment documentation.
When this was going through Parliament, I put all these concerns to my MP at the time - his response was a variation on "If you have nothing to hide, you have nothing to fear".