In practice I think a jurisdiction defence (no representation in Brittain, nor do we explicitly target British users) would also be enough. The onus on the $UKGOV to prove otherwise is so big....
No: if the forum is /accessible/ from the UK, then you have to comply. The onus is on (collectively) /you/ to demonstrate that even a single para of procedures is in place by March: if HMG challenges you and you can't say "this is what we did before the required date", then you lose by default.
Look, I've been through some of this crap over the last couple of years: discovery (i.e. both parties digging out all relevant correspondence) is taken seriously, since if there is an agreed list of documents (which explicitly includes "social media postings", which is probably how discussion between forum moderators would be characterised) then the judge can read them in the comparative comfort of her chambers during the first day or so of the hearing.
If you can't produce some sort of policy document then you'll lose, you'll be fined, and (worse) you'll have to pay HMG's expenses.
If you can produce even a very vague policy document, then unless somebody's been induced to do something very illegal or (self-) abusive by discussion in the forum then you might get your knuckles wrapped but- with luck- no expenses order will be made.
MarkMLl