From what I’ve read so far, I would either have to run the forum as a sole trader or, I think more relevantly, as an ‘unassociated association’.
I help run a server co-op here in the UK and we pay our hosting costs out of membership subscriptions. We don’t seek to make a profit other than for contingency (when we used to co-locate and needed new hardware, for example). Perhaps that’s similar to your situation?
If so, we set up as a “Partnership” (i.e. an unincorporated body) under UK law. We don’t have to submit accounts, but do submit an annual tax return. I can ask our treasurer what that involves, but I get the impression it’s very simple.
BTW we were VAT registered for a number of years as some of the members were too so it made their subscriptions cheaper. But we de-registered recently as it wasn’t worth the hassle after a while.