I also live in an upstairs flat in London with no downstairs hallway to leave a buggy in. I've had 2 kids and had to carry stuff upstairs every day. Even so, I'd definitely say get a pram.
I have a Beco Butterfly sling, which is like an Ergo. great now my DS is 6mth or so, but when tiny he didn't seem comfortable in it (they have to have their legs really wide apart).
I can't imagine not having had a pram for my two DCs, especially for the following reasons:
In bad weather it's lovely to just whack on the raincover - also in windy conditions it keeps them sheltered
when you're going out for the day its tiring to carry baby all that time, plus they don't get to stretch out flat at all
As someone else said, you can have the baby sleeping in the pram next to you while you sit and have a coffee, lunch etc.
Don't forget you have stuff to carry as well, and pushchairs are very useful for carrying shopping and the changing bag.
Finally when you get on the bus or tube, and are carrying on the front, sometimes you feel you are squashing babys legs when you sit down.
Good luck anyway!