Ok, This site says:
When we talk about ability, we mean two things.
First, we mean general ability. This is something that once you have learned you can do any time you want, like being able to read or swim or speak a language, for example.
The other kind of ability is specific ability. This mean something that you can or can't do in one particular situation. For example, being able to lift something heavy, or find somewhere you are looking for.
can / can't (for both general and specific ability)
I can play the piano.
She can speak English.
He can't drive he's too tired.
We can't come now.
could / couldn't (for general ability)
I could read when I was four.
She could speak French when she was a child, but now she has forgotten it.
He couldn't dance at all until he took lessons.
My grandfather couldn't swim.
was able to / couldn't (for specific ability)
When the computer crashed yesterday, I was able to fix it.(not 'I could fix it')
She was able to pass the exam, even though she hadn't studied much.(not 'she could pass')
He called us because he couldn't find the house.
I couldn't open the window.
See this conversation from an English TextBook-Solutions 2ndEd
A: Where were you last weekend?
B: You weren’t at Sam’s party
A: No, I couldn’t go. I was with my parents. We were in London all weekend.
B: Really? Was it fun?
A: Yes, it was. Look at my photo of the London Eye.
B: It’s great. I really want to go on that. Was it good? What could you see?
A: It was fantastic. We could see the whole of London.
There is 1 expression in the above conversation "We could see the whole of London.". It sounds wrong to me because it is a specific ability & therefore we need to use "was able to"
So, It is better to say "We were able to see the whole of London"
Is "We could see the whole of London." wrong?
Note: Even Oxford expert says it is wrong to use "could" for specific ability. See this video