Why is there this fake window in the opening of 'Bottom'?

It may have been an improvised shot with no real connection to the show

Rik references the window in an interview quoted in this BBC article:

Mr Mayall once told the BBC that discovering that the original bench had been removed was the "worst journey" he had ever made.

"If you were to come to the end of King Street where it meets the big roundabout to get to the Hammersmith Apollo, you'd find that same bench on a traffic island - until one day they took it away and put a pelican crossing in its place.

"My old bench. Me and Eddie's old bench, that's what I miss the most," he said.

He also discussed the spontaneity of how the scene came to be filmed.

"There's a very clever man called Ed Bye who's a very talented director. He said 'Alright boys, we've got to get a title sequence together - come on, come on, come on'.

"We ran around looking for somewhere to take shots, and that's when we shot us on the bench.

"And they were just building this enormous building and there's Richie and Eddie looking out of the window of a half-built place, which is also in the opening sequence.

"So I think the history of Hammersmith is very much in the Bottom show."

I’m not sure of the smell underlying meaning in the imagery, not convinced there is one. But I do know about the ‘fake window’ which was located to the east side of where the Broadway Shopping Centre is now. There used to be a small temporary bus station below and to the left, where I waited for my bus home from school in the 80s and early 90s. The series started just as that new shopping and transport hub was being built and the fake window was I believe just a display to show what the facade of the building would eventually look like - it was there for a year or so while the whole area was a building site.

As far as the original Eddie & Richie’s bench goes, it was never located in the spot where it was filmed. For filming purposes a bench was put on the ‘wrong’ side of the road barrier I.e. actually in the road (presumably so they could get a long shot, with the camera approximately located out the front of the Odeon (as it was then), with traffic interjecting between the lads punches.

There’s now a memorial bench for Rik on the same traffic island but not the same position and the bench is a more decorative design but a nice touch.

Fun series.

What we know for sure - the intro is to first give the impression they live in some spacious luxurious place from where they watch other people.

My reading - this is a jab at trope "posh people watch and talk about other people" (kind what we see with Statler & Waldorf in Muppets). In intro we see that Richie and Eddie are more interested in violence against each other than commenting other people lives and THEN the trope is deconstructed with "this is not upper east-side Westminster condo".
And then the title is shown - BOTTOM. And that tell you all about the show.