A word in support of James for a change.
The limited liability is just that - to limit his liability to the capital of the company, which can be kept at virtually nil. So they cannot go for his personal assets like his house.
But the forum can be closed by a successful libel case, and it depends on if he is culpable personally in the libel as he could still be sued as an individual, he should avoid naming anyone as the responsible publisher in the forum as they can be gone for individually as well, Ltd company or not.
Also with no win no fee lawyers constantly punting for business, running a forum is a risky business. But it should always be remembered the publisher of a libel (forum owner and interestingly responsible mods) is only liable after the fact i.e. the individual who made the post in the first place.