Is there any other information you can recall about the game? Platform, character design, art style, etc?

I FOUND IT! I ACTUALLY FOUND IT! I had exhausted all my more targeted ways of finding it and resorted to Google images searching for "point and click game bar" and found a screenshot I recognised.

The game is Mudlarks.

Sorry for the couple of details I misremembered, and thank you to everyone trying to help.

Do you recall the platform(s)?

I also remember a scene on the street outside the missing friends flat (apartment) and the protagonist talking up to a woman hanging out an upstairs window. Think he convinces her to throw down the spare key for his friends flat.

I find AI to be really good at this kind of stuff. If you give it as much detail as you can, including random tidbits, it can often find exactly what you're looking for. I've done it a few times, and it's always found it with, what I believe, was not very good information. If it doesn't give it to you, just keep adding random pieces of information.

What you've provided doesn't quite seem like enough, because I tried it and didn't get much luck. The best it came up with was The Silent Age. Try answering these questions:

1. Was there narration
2. Was the protagonist male or female
3. How long ago did you play it? Like, was a game from 2 years ago too recent?
4. Are you confident it was from the last 10 years, or did you just play it from the last 10 years?
5. If it was point and click, were there normal animations? Like the character walking over to the thing you clicked?

I've never played it, but perhaps Mask of the Rose?

Unlikely to be it since it's nowhere near from the last 10 years, but CITY 2000 seems like it could be similar at least artistically?

I think the best I can recommend is looking through Steam, searching for "London" and the mystery genre. I didn't quite catch anything there that fit at a glance, but maybe you will. Similarly could be done on GOG, since it sounds like it could potentially be an older game? Or itch, but maybe the best way to search for that would be by googling london missing friend mystery site:itch.io.

I'm assuming a modern setting, with no supernatural elements and the mystery genre, so that's the best I could do. It's going to be very hard to find something without some details being fixed. Point and click? Photos or isometric? Is the player character visible? Do they have any identifying details? Does the pub have a name? Anything like that could do a lot.

You say you watched someone play it on youtube then you might be able to search your youtube viewing history?