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

I don’t believe it would target any of the fediverse currently. No instances have significant volumes of users or target markets. This is designed to target facebooks, tik-toks, and twitters. Services that do influence populations. Essentially, making these services actually responsible for their algorithmic output and akin to publishers in the UK

Simple. Don’t be a company. Companies exist to sell stuff: the fediverse doesn’t. There are a few other structures in the UK alone to circumvent this potential law, which is designed to combat large social media companies. And it won’t affect outside the UK either, and with brexit - will anyone else bother enforcibg?