Never worked in the City but I have had many dealings with them. I worked for a major UK Bank and we had our own Private Bank and Investment House in the City. Whilst they were now operating under the umbrella of the parent brand, they were originally old named city business's and, internally, they retained their old school culture and of course their magnificent buildings in the St James region of the City, that resembled old gentleman's clubs.
I would often accompany clients down there. It was generally the sales people (relationship directors) I'd be dealing with but sometimes, especially where there was big bucks involved, we'd get to see the senior guys who actually run the funds. These were god like figures, paid serious dosh, invariably male and always complete arseholes.
The hospitality was always spot on and no expense spared. When I wasn't required to return to Liverpool with the clients, I'd sometimes book overnight down there and was well looked after. The nights were very boozy, so the show was realistic in that regard, although personally I never saw any use of the white stuff but was equally sure it went on. More than once I missed my last train home when I hadn't planned to stay over (think it used to be something ridiculous like 8pm) and was put up by one of the staff.
I probably went down there maybe 20/30 times over a 10 year period when I was in my 40s. The guys and gals down there were a lot younger, 20s and 30s. I was always laid out for days afterwards but these guys just went back to work the next day and got on with it. Amazing. lol
I did get to know some of the people really well and over alcohol they quite often tell you more than they should, but I probably don't remember half of it with being so pissed myself. But I do know that they are very results driven places, competition between teams and individuals, and even companies, was massive, and they were all hugely ambitious, but deep down quite unhappy people. I got the impression bullying and sexism was pretty rife but accepted as part of life.
At the time I used to really enjoy my jaunts down there. They were a glimpse at how the other half live. But thinking back now I had nothing in common with them really, being just an ordinary scouse lad, other than an enjoyment of getting pissed and dining out on good food.
Edit
Oh forgot to mention, The women are also much fitter, not to mention dirtier, in the TV version.