The country in your Samsung account cannot be changed unfortunately.
Google allows you to change the country, but Samsung does not. Only way is to create a new account ( you will loose all the data and purchases though).
Can you use Samsung wallet in the UK without changing the country? I think the Samsung pay UK works differently.
There is this link from Samsung:
Not every account works like that mate. I registered in Nigeria and now in the UK but most of the apps I want to use (such as Google pay, Tesco club card etc) are not supported. Each time I want to download anyone, it shows an error message that the app is not supported in your country.