Usage of the app isn’t mandatory, you aren’t being discriminated against, you’re using a non-UK device meaning some UK only services might not be available to you.
It is saying if you are not from the UK and therefore don’t have a UK phone you are not allowed to use the app. Why else is the app so restrictive? So it is discriminating against non UK nationals.
But not all UK based users will have UK Apple ID so again how is that not blatant discrimination? Why not to change the settings to ensure inclusivity as opposed to continually trying to defend this?