That's what I also fail to grasp.

If things get serious and for example the WhatsApp, Telegram and Signals of the world have to disable e2ee to keep operating in the UK, I guess they could just leave that market before compromising data privacy of their users worldwide.

I don't see this "ending encryption worldwide" thingy happening, at all.

But they could dig themselves a deeper hole after Brexit, that's definitely possible and wouldn't surprise me a bit.