I don’t believe that this would cause any legal problem; I came across the following link, with the usual caveat of not blindly trusting anything you find on the internet, it does appear to set out the legal responsibilities.
https://internetlawcentre.co.uk/lega...tors-in-the-uk
Basically, operators of a website which allows for user-generated content are not liable for it, as long as once notified that said content is potentially illegal or defamatory, that they take action within a reasonable time - and that doesn’t necessarily mean that the post is deleted.
But pre-moderated posts is a logistical issue rather than anything else. When I logged in, I was told there were 23 unread post since my last login, about 20 hours ago. So let’s estimate 30 new posts a day (and I’m sure that one of the mods or admins may well, pitch in and claim that the actual average number is far higher). But someone has to be on standby to read 30+ posts a day 24/7 and make a judgement call as to whether or not they are appropriate, and allow or deny them - it’s almost a full-time job!
But, ultimately, this is a private forum, the admins and mods are free to run it however they see fit - hypothetically if they only want to allow left-wing leaning or right-wing leaning views and delete all others - that’s their prerogative. I think (and just to disclose I and not a lawyer), I suspect they would be fine. Of course, in reality, the said forum just becomes an echo-chamber and no longer functions as a way of exchanging ideas - cough ‘Truth Social’, cough. Or try criticising Elon on X and see how long your posts and account lasts!
In theory, all posts being pre-moderated before being made visible on here is probably viable. But X, Facebook, Instagram, etc. which are many, many, many orders of magnitude larger, no it’s just not viable?