In the EU (which I know no longer includes the UK), both Apple and Google will have to allow sideloading in their operating systems by early 2024, because they* are expected to be designated as gatekeepers under the new Digital Markets Act no later than early September 2023, and after being designated they’ll have six months to comply. So the feature to sideload will continue to exist in Android and will be added to iOS, although they could choose to enable it only in the EU or to block it in the UK.

*If you choose to fact-check this, be aware that technically the European Commission will be designating Alphabet, Google’s parent company, rather than one of the multiple subsidiaries with Google in its name. So the official EU communications about this may not mention the word Google.

Remember that the Play Store uses encryption to download apps to the phone. If the law is enacted then this encrypted channel will technically need to be compromised too, which Google might also object to. If Google withdraw the Play Store from the UK, then what are they supposed to do?

This law doesn't just apply to messenger apps. It applies to almost every single act you perform on the internet. Everything will be mandated to be broken, including your browser. If you run a web server using https, you'll be breaking the law. If you ssh to another computer over the internet, you'll be breaking the law. If you connect to a VPN to access a work/university/school network or indeed a commercial VPN, you'll be breaking the law.

Stopping sideloading will never fix this issue, just having one non-cooperative external website would mean the scheme falls apart, you need to start blocking at the network level and aggressively pursuing people who bypass those blocks, making an example out of the first one that hits courts would likely be enough to scare all but the most dedicated out of such pursuits.