My ugly solution to this problem is to have a free Oracle Cloud VM in the other country that I use to run a VPN (Oracle provides instructions [1]). I then connect to this using OpenVPN on my phone, which allows me have a Google account that thinks I am in the other country and so allows me to install apps that are restricted to that country. I don't have the VPN connected all the time - only when I want to access the App Store using the Google account that I have for the other country.
To be a little pedantic, your solution is a solution to your problem, but only a fraction of the problem you're responding to. Your VPN won't help access the UK apps that require a UK phone localization if those same services aren't also available in the region of your VPN exit node. And since he's talking about UK-specific apps and services, VPNing his US phone back to the US isn't any help.
This is essentially what's known as 'digital migration'[1] in mainland China. Many streaming services aren't available in China; foreign companies that do operate here often have their features reduced. So, apart from buying foreign SIM cards and using multiple Apple accounts, we typically subscribe to something called 'airports,' which provide standardized, open-source VPN protocols with servers (called 'nodes') in various regions. Besides bypassing internet censorship, these nodes often use residential broadband and specify which streaming and LLM services they can unlock.
[1]: https://blogs.oracle.com/developers/post/launching-your-own-...