I sympathise with the OP because at some point everyone becomes too old to deal with the headaches of running a community. I have no opposition to their choice to shut down the forum. I just don't believe liability as a result of the new bill is the reason.
Rightly or wrongly, limited companies in the UK provide a high degree of protection for wrongdoing. Defrauding HMRC out of hundreds of thousands of pounds and suffering no consequence is happening day in day out. An Ofcom fine is nothing by comparison.