This is a whole story unto itself. I (and a bunch of other small-site operators) actually got the chance to ask Ofcom directly about how small a site would have to be to declare itself out-of-scope of part 3/5 services, and in short: the law doesn't specify, Ofcom declines to say, and it could be as small as "a one-man band". Ofcom has indicated that they're interested in pursuing aggressive enforcement against small service providers (read: web sites). I wound up concluding that the forum I help run could be defensibly out-of-scope, but aphyr.com gets significantly more traffic, and I'm trying to limit risks.

I think in total I spent something like 80 hours reading the legislation, working through thousands of pages of Ofcom guidance, and corresponding with Ofcom trying to figure out whether and how compliance was feasible.

https://blog.woof.group/announcements/updates-on-the-osa