If you subscribed to BritBox through your Telia TV4 Play account, you won't be able to use that subscription to sign in at BritBox.com or on the BritBox mobile, tablet, or TV apps.
You'll only be able to watch BritBox on the platform where you signed up.
For additional context:
Since you subscribed to BritBox through Telia TV4 Play, you won't be able to log in at BritBox.com using your Telia TV4 Play account. Instead, you’ll need to watch BritBox through the Telia TV4 Play app.