This is already impacting futurology.today - one of the Mods is British, and because of this law doesn’t feel comfortable continuing. As they have back-end expertise with hosting, if they go, we may have to shut down the whole site.
How easy is it to block British IP addresses? Would that be enough to circumvent any legal issues, if no one else involved in running the site is British and it is hosted somewhere else in the world?
So with british law, the intent of the law is as important as the written texts. Listen to the debates which can and are used by judges from the commons and the lords to decide upon intent. It’s not for tiny forums, but I’m also not a lawyer. Significant most likely relates to not just user count, but also other reporting from other media, it’s significance of significant users, anonymity, and ability to break bigger stories. Try codifying any of that (and more!) in a law