It's a bit later nowadays, at least in the normal sorts of places you'd go out (Soho, Shoreditch, etc.). Though, what are you doing working until 11pm?
In any case you'll still find the roads can be busy at all hours. The night is used, as you say, for a lot of infrastructure type stuff which means the main roads are still busy, getting the city ready for the next day.
I no longer do - working in the City wasn't for me. And neither was living in England. A week in the summer and over Christmas turned out to be just the right amount of blighty for me.
In any case you'll still find the roads can be busy at all hours. The night is used, as you say, for a lot of infrastructure type stuff which means the main roads are still busy, getting the city ready for the next day.