A population of 9 million but big tech's allergy to scaling customer support means they don't pay a local person to adjust things for local expectations.
They have a large office in London, so their own staff put up with it.
I once met a Googler who had previously worked on maps. I made a few grumbles (railway lines at the time were almost invisible, but very useful for orienting yourself when you exit a station, gardens were grey yet parks were green, "Transit" only shows the London Underground so it misses most of the local rail travel routes in South London, etc).
He agreed with everything, but said the staff working on Maps were all in California and wouldn't care, if they could even be made to understand the issues.
Actually the teams that would implement those fixes are not really in California (rather Seattle, Sydney, Tokyo, among others), but the problem is more the diffusion of responsibility and lack of ownership. Teams all have different metrics they're trying to drive or features they're focused on, but no one is responsible for the overall quality and user experience of "Google Maps" as a whole.
All of the things you mentioned have internal bugs, pitch decks, or maybe even patches pushed by individuals or whole teams. Likely even directors or VPs agree. But the organizational friction is massive and it's very hard to ship anything unless it's crystal clear that it is going to directly move a metric that is a focus for this year. And even then, if launching this feature touches multiple divisions, it still might not happen just because of the massive coordination effort.
That said, the green coloring was probably fixed in 2020. Transit has had a lot of work in the past five years, so maybe a chance London overground rail is fixed? The transit line rendering is still terrible, so you should probably use Apple Maps for that—theirs is beautiful.
The discussion was around 8-10 years ago, so I'm probably mixing it up with Seattle.
Scrolling around the UK with the transit layer active, I only see the complete metro/light rail/tram systems of London and Sheffield (incomplete/missing "heavy rail" for both). Newcastle has one metro line, but not the others. No other city's lines are shown even though the stations/stops are often mapped. (I checked Glasgow, Edinburgh, Manchester, Birmingham, Nottingham, Blackpool.)
In much of South London "normal" trains are the primary public transport. The stations are marked, the railways shown in light grey, and there are usually even timetables listed [1] but there's no "Transit" line shown. These lines and services aren't shown anywhere in the country.
[2] is a public transport view of OSM. The colours are poor, but the orange lines are mostly missing in Google.
[3] is easier to read but only a railway map, though over 90% of lines in Britain will have passenger service. I generally use this if I'm planning a flexible trip somewhere unfamiliar.
I see some long-distance lines are shown in Japan, though surely only a fraction of what exists. They also get a darker shade of grey, so there's been some local effort there. There are still missing local lines in Tokyo though. I know Copenhagen, and there's been a significant improvement recently! The city/suburban trains are now shown. The regional trains are still missing though, which includes the train from the airport to the city centre.
Native SE Londoner here. IMO Google is accurately reporting the transport in the area, it's not their fault that there is a shocking lack of it.
Lines like Victoria-Orpington and CharingX-Hayes are not the metro and probably shouldn't be highlighted in the transport layer in my opinion.
The solution you are looking for is TFL taking over the suburban southeastern lines. That's what happened in my adopted home of NE London and today they are orange lines on the map. We also got brand new trains and 4tph on Sundays!
But why would they? We have to use it anyway.