here’s the thing though, you wouldn’t need to do that second part. You only need to know what the relative time for london is in the event that you fly over there, or something, and even then
What? Sorry, I must be misunderstanding your viewpoint here. People interact all across the globe all of the time; it’s important to know what part of day it is in the different places for all of that. You want to call someone in Singapore? It doesn’t help to know their clock shows the same time as you, you need to know if it’s the middle of the night, or maybe it’s likely lunch time etc. That’s why you need to know the offset from “your” time.
And you glossed over everything else… I’m not talking about movies for no reason. Movies tend to need to convey lots of information in a short amount of time so it’s a useful example of the differring amounts of information that can be communicated when we all share cultural understandings of things. If 3am means essentially the same thing everywhere that’s super useful in communicating all sorts of ideas.