To answer your question first:
Landmarks:
1. London Eye
2. Tower Bridge
3. Big Ben
I think these are globally the most recognised and spell out London the best.
Vehicles:
1. Routemaster
2. Black cabs. I know these won't be player controlled, but you really need to have them.
3. Emirates Air line, an new cable car that will be opened before the Olympics (ahem, hopefully). To be honest, I'm just trying to say something different, but it would be cool to have that.
Oh, and when it comes to London and public transport, the star really is the tube map! It would be absolutely stunning to have the ability to create a map of your lines in the game in the style of the tube map.
As somebody who lives in London, I wholeheartedly agree with the request not to make it a generic Middle European town, that would feel completely wrong. One thing I have never understood about CiM is why all buildings have generic names. I understand the limitation of resources so that not every building can be designed separately, but at least they could all have different names, right? So let's not have Railway Station, Railway Station, Railway Station and Railway Station, but Waterloo Station, King's Cross, Victoria and Clapham Junction.
Some things that could make it feel more like London:
Street furniture:
- red postboxes
- red phoneboxes
- blue police boxes
- Belisha beacons
wiki, the lights at zebra crossings
- Boris bikes
- people everywhere
- stands selling Union Jack themed everything
- roadworks
Traffic:
- the afore mentioned black cabs
- tourist coaches
- open top hop on/hop off buses
- white vans (preferably dented)
- Rolls Royce
- expensive sports cars
- Reliant Robin for earlier scenarios
Also, most of the through roads should be classical high streets, so lots of small shops at the ground floor of all the buildings, with apartments on top.