I recently moved to Sky for my internet service. discovery+ was part of my BT package (but it is apparently a fixed term subscription so cannot be stopped early etc) but I can't seem to find out when it ends.
Sign in to the discovery app
Clink on top right corner
There you'll find manage subscriptions
All I see is this

If you're billed through BT it's likely once your broadband finishes your tv will as well.
Like it says check it on my BT.
Just click my products.
Yes once you cancel your account, Discovery+ will automatically cancel at the same time.