Is it Uber? It's been happening only in the past year or so, and increasingly.
Bog-standard saloons, average 3-6 years old, sit outside on a single yellow by around 1am, sometimes now four or five of them in a row. Mostly with the engine running, a lot of which are diesel though some are hybrid Toyota types.
Always just a sole occupant behind the wheel, some of whom at times take/make phone calls. Yet never a TfL minicab sticker on the back.
I'm a night owl but this is a presence that's also being inflicted on sleeping neighbours. I first noticed it at weekends; now it's becoming a midweek phenomenon too.
Engine running ? I think it is not legal to have engines running whilst stationary , it was a real problem where I live , I would go out and tell them to turn it off. Years ago there was a van parked up right outside my door at 3am I asked him to turn it off, got the normal response, only be here five minutes etc. I left the front door open went in the garden got a ten pound sledge hammer and the sight of a large woman swinging a hammer over his head persuaded him his five minutes were up!
Seriously go an tell them to turn it off else police will be called, word will get round.
I agree with Jan the old one and if I were you I would take down the registration plate numbers and time of day/night and report them! The air pollution in London is bad enough as it is and to leave engines running whilst stationery is just not on!
You should film of all of them together and put a leaflet on their windscreen telling them that... "Stationary idling is an offence under section 42 of the Road Traffic Act 1988" and that you will be reporting them. Try it and see if it works.