Turns out it’s just legacy from the original escalator design.
“It all dates back to when the first successful escalator (there was a failed attempt to build a spiral escalator beforehand) was introduced on the tube in 1911 at Earl's Court Station. Alighting an escalator was different back then to nowadays. There was a diagonal partition — beneath which the stairs disappeared — that shunted passengers disembarking to the left. So it was decided that those walking up the escalator should stand on the left, otherwise they'd have to cut through a line of those standing. And that would've been mayhem.”
In the US, we walk at traffic when there are cars on the road but the passing lane is the same (left) for driving roads and for pedestrian multi-lane "roads" like escalators.
“It all dates back to when the first successful escalator (there was a failed attempt to build a spiral escalator beforehand) was introduced on the tube in 1911 at Earl's Court Station. Alighting an escalator was different back then to nowadays. There was a diagonal partition — beneath which the stairs disappeared — that shunted passengers disembarking to the left. So it was decided that those walking up the escalator should stand on the left, otherwise they'd have to cut through a line of those standing. And that would've been mayhem.”
https://londonist.com/london/transport/why-don-t-we-stand-on...