I don't live in Lahndun. You can put my answer down to my provincial domicile, if you like.
I always take the lift in department stores, as do all the other pushchair and pram pushers I ever see. I've never seen anyone taking a pushchair up them thar movin' stairs!
Despite this, it's still possible to use the lift, contrary to one poster's concern that lifts would be unusable, if all pushchair pushers tried to use them, earlier on in the thread! The lifts just go up. And down. And up and down. More often. It's not like buses, that are timetabled to come at pre-arranged times. With lifts, there are buttons, and you press the button to summon the lift. Crazy, eh? Perhaps it's different in London... I hear the buses are better, so maybe the lifts are worse in M&S. Makes sense, right?
More seriously, and less snarkily, I take the notices forbidding pushchairs as applying to everyone, not everyone-except-me, so I follow them. Besides, I suspect that if I took the escalator, it would be quicker for me, at the expense of holding everyone else up