Anyway, my main experience of Matrix is "failed to decrypt message". It's... not great. I wish it were better.
There are a few IRC clients that support OTR. irssi-otr is one [1] weechat-otr is another [2]. Pidgin though I have not used it in a very long time. Hexchat using an always work in progress plugin. There may be others.
OTR could use some updates to include modern ciphers similar to the recent work of OpenSSH but probably good enough for most people.
E2EE aside having chat split up into gazillions of self hosted instances makes it much harder for chat to be hoovered up all in one place. It takes more effort to target each person and that becomes a government scalability issue. Example effort: [3]
[1] - https://github.com/cryptodotis/irssi-otr
[2] - https://github.com/mmb/weechat-otr
[3] - https://archive.ph/4wi5t
Additionally, OTRv3 does not allow multiple clients per account, which makes it unusable for anyone who wants to chat from two devices.
Practically speaking, I would just ignore this requirement. The UK government has no jurisdiction on this side of the pond.
In that kind of environment, end to end encryption really doesn't add value.
Even without registering my nick, I would expect a modern protocol to keep my pm communication private by default.