Unless they have some connection to the UK outside of some users or they have similar laws in their home country they're probably not going to bother. The trend is saying your law applies globally, in practice that's not the case unless you're multinational. The UK government will probably end up having to block them if they don't comply.
They tried to pass a similar law in California with the backing of Baroness Kidron and it got struck down. So Ofcom can sit and spin here when it comes to small forums.