Hi,
I'm trying to do the same thing, although I'm going more for a British seaside town and surrounding countryside rather than London, which most people are.
I'm fairly new to modding having got my PC during lockdown. To give mine a British feel I've downloaded mostly unmarked British police cars to use as civilian cars, there's lots of Audis, Skodas, Fords, Vauxhalls, Volvos, Land Rovers/Range Rovers, BMWs etc. and they all have British registration plates. You get the odd extra aerial or blue light behind the grille but its mostly not visible. There's also a handful of non-police British cars about. A Peugeot 308, a MK3 Focus estate, a Hyundai I30 and a BMW 330I being the only ones I can think of off the top of my head.
There's also a fair few British vans and trucks about. There's a Vivaro van with British plates and a Royal Mail skin, loads of Sprinter vans including a G4S Security van, a Vito van and a few Ford Connects, as well as an AA Volvo truck, a Mercedes Actros recovery truck and a Scania Recovery Truck. I've also got a British fire engine and ambulance, British marked police cars that there's loads of, a London Taxi and Stagecoach/ National Express busses and coaches and an Actros Trashmaster. I've used a few other European style cars for the remaining vehicles, although these don't look quite as good as they don't have the correct licence plates even with a mod in use. I'd change the licence plates myself but I'm not sure how. They still look better than the in-game American cars though in the British setting.
At first I replaced all the cars in the game and my game kept crashing so since I've made sure most of the cars I installed were less than 10mb and it still crashes but I get about 30 mins gameplay in between crashes now.
In terms of other stuff, I've added the freightliner train from here and the British roads, street signs and props mods. I've also installed the project London mod. The shops on there are good but there are some London specific items like Wembley and tube trains and as its all one file I'm not sure how to remove them. There are also mods for Asda shops, real petrol stations, northern vegetation to remove the palm trees and cacti, a Premier Inn and NCP car parks which I've added.
Most of my mods are either from here, GTA Inside or GTA 5 mods if you want to search for them on there as I don't still have the links. I just need to work out how to get LSPDFR and Ymap mods to work now and ELS as my emergency vehicles currently only have sirens and no flashing lights. I also need to do something about the north of the map to get it to look a bit more British and get rid of the American flags and American style billboards and also replace a lot more of the American shops. It looks like there was a British shops mod but this sadly seems to have been removed from the GTA 5 mods website.