You can set the app store region to Belgium so that you can get hold of the Belgian version of the app, but it does require you to have an address and payment method that allow you to use that Belgian app store. The instructions are here:
https://support.apple.com/en-us/118283 (Don't forget to remove the current version of the app before switching).
If you don't have a Belgian bank account, have a look at Wise. It is an online bank that allows you to have multiple currencies linked to the same account. It may allow you to use a Belgian (or EUR capable) account that you can set as a payment method in the App store. You can try and see how far you can get without a payment method in your Belgian app store, that is not something I have tried. It may work that way as well if the app is free.
That leaves the issue of a Belgian address. Do you have a friend or family who wouldn't mind you using their address? You can argue the legality of doing it this way, but once you have the Belgian version of the app installed and working, you should be able to switch back to the UK version of the app store and continue to use the app.
All a bit of a faff.... but if it enables you to keep your pension it may be worth it.