shows aren't available in my country
If the message is accurate, then are you in the country that matches where you subscribed? BritBox (and many other content providers) have content licenses for a single country, which prohibits them from streaming abroad.
If the message is not accurate, then verify your IP address. Could that cause (some) content providers to think you are abroad? E.g. is a VPN active, or could your ISP have distributed you with an IP address that might point to a different country?
Location lookup for IP address services: [1] [2]. (I found these pretty accurate.)
If none of the above applies to your situation, then please contact the Apple Support staff for further assistance.