Isn't this situation the entire purpose of Signal..? Don't they already operate in the most authoritarian countries in the world? I don't understand why they would walk.
Apps like Signal will then have to make a choice. Either publish an app that does not use E2EE or get removed from the app stores. What Signal are saying is that they would accept the ban rather than publish an app with weakened encryption.
Apple users will not be able to make their own choice. Android users will be able to make their own choice thanks to side-loading.
Recent update on WebKit about PWA, however, makes anything possible now on iOS devices.
https://webkit.org/blog/13878/web-push-for-web-apps-on-ios-a...
Technically you need browser anymore to run proper messaging apps.
I believe the problem is, UK is extremely powerful in terms of asking “please ban this and that app from app stores”, so they can effectively block it (unlike say Russia, which tried to block Telegram and failed spectacularly).
You can use leaving as a point of leverage in a country that allows for open debate about end-to-end encryption. You can claim that being in the country is at odds with the country's principles and threaten to pull out, or actually pull out, to make the users in that country aware that their basic privacy is being pulled away from them.
In a country with an authoritarian regime, being an option under the radar provides may be a better option.
In UK if Signal has any offices or employees, the UK government can force Signal to either put a backdoor in the product or move out of the country. The second choice is what Signal said it will do, it makes perfect sense - leave instead of bend over.