When I had an Amazon Prime Video subscription, almost everything I wanted to watch had to be paid for separately, yet had Prime Video labels all over the place.
It took me months before I realised that Prime Video wasn't actually getting me anything I wanted. I needed to pay outright for each season of what I wanted, and Prime Video didn't change anything about that, such as prices or what was available.
When I figured that out, I dropped the subscription. I still watch things on Amazon Prime Video, without the subscription to Amazon Prime Video. Yes, confusing.
They also have prime video, which is a subscription service that is included with an Amazon Prime subscription (although you can also pay for it separately if you don’t want the other prime benefits)
With prime video, you can watch all of the Amazon originals (like the expanse season 4), and a subset of their entire collection that they license from other publishers.
You can filter to just display shows and movies that are included in the prime subscription, so you don’t keep picking a movie or show and find out that it is not a part of prime.
What I found is that on "Amazon Prime Video", almost everything I wanted to watch needed to be paid for because it wasn't part of "Amazon Prime (Video)".
It took me a long time to realise that "what you can get on Amazon Prime (Video)" and "what you can get on Amazon Prime Video" were completely unrelated things, and that I could access exactly the same things that I wanted, at the same prices, from "Amazon Prime Video" while stopping my subscription to "Amazon Prime (Video)".
Amazon's documentation and labelling from the set-top-box app did not make this especially clear.
The Expanse last seasons were a notable exception. I'd already seen the earlier seasons on Netflix a long time before, and it was very annoying to be unable to watch the second half of the story So I was delighted when I looked on Amazon, and watched them just before cancelling my Amazon Prime (Video) subscription. Now I just watch Amazon Prime Video.
It is quite possible I've got the names of these things completely wrong. I don't really pay attention, because I very rarely use Amazon's consumer services, except via set top box. (I use AWS but that's completely different.)
Amazon still lets you stream content it doesn't own (like how iTunes Video lets you stream TV seasons), you just have to pay for them.
If you have Amazon Prime (and are the primary owner, which I'm guessing you aren't since you can't access it), then you get Prime video.
So I, too, have Prime since my brother added me under his account but he only has access to it since he's the owner.
I would echo what the other commenter said... are the the primary account holder or added to another account?
But it might be different with the Expanse in the UK.