Discovery+ without TNT sports is super cheap. Only £6.99, its only when you add TNT sports is when it’s stupid money, unless you are a BT/EE customer.
Still half the price of doing it the other way, though, if I have correctly interpreted the labyrinthine pricing options….
Don’t know where you are getting your information from. But to get Discover+ you do NOT need Amazon Prime.
https://auth.discoveryplus.com/gb/product