Edit: there's a list of the Category 1 services at
https://www.ofcom.org.uk/online-safety/illegal-and-harmful-c...
I.e., Facebook, Instagram, Pinterest, Quora, Reddit, Roblox, Snapchat, TikTok, Whatsapp, X, YouTube.
Plus others at the Category 2A or 2B levels.
Edit: well, it says they are "Providing user identity verification options." Unclear to me when they are supposed to get users to verify identity.
I presume this is the extra obligation starting in 2027 that wikipedia mentions.
I wonder how simplex.chat would fair if it ever got popular enough, they're based in the UK and likely the most private messenger ever made, because the app introduces zero metadata. It's decentralized, but they have servers and the dev is based there...