There are limits, a 3G cell can only have 32 neighbour relationships for handover...
Lose 2 for it's other sectors on 3G (assumed 3 x 120 degree sectors on 3G and 2G and 4G)
Lose 3 more for 2G
Lose 3 more for 4G
Now down to 24 slots for neighbours, tall building? (like most in Docklands...)
you might have in-building solutions on various floors who need a handover to the external macro network pointing at it for people who work near windows / exit the building (ground floor only!)
Take One Canada Square in Canary Wharf, 50 floors with metalised windows so some move RF is reflected back so in-building systems required.
But around One Canada Square there are 10 buildings above 30 floors each with the same problem, your remaining 24 neighbour options ran out on the first building, what do you do for the other buildings in the area?!
The you have a problem of frequency reuse, each operator only had X amount of 2G/3G/4G spectrum, EE has a lot of the above but its still not enough, another 100Mhz 4G suitable spectrum from Ofcom would be handy....
So dropping in more cell sites can often make things worse, its a very difficult task to design a mobile network that serves the usage generated in such a tight location as Canary Wharf, 'The City of London' is easier because of smaller buildings and spread over the square mile.
And as James said, yes all major cities with very dense business districts have this problem.
Paul