locked Session Messenger.
Also owned by a company, an Australian company, which have been a discussion topic before as Australia have similar laws here as UK. The app itself is open source, just like Signal's is, which gives a lot of safety and guarantees here, but they own the network and can control what apps connect, so isn't really decentralized. Not in the sense eg Matrix is, where both the app ecosystem and network itself is decentralized (fully federated).