Good question.
The other day Merseyrail Tweeted that there was a power outage and they had to close one of their Liverpool stations.
A passenger queried it saying that it was just the escalators weren’t working and was told that they are not aloud to keep the station open if the escalators fail?
Saperstein
It’s not the case that there must be stairs - many London Underground stations have no means of access other than escalators, including for staff.
In some cases a stairway was kept when only two escalators are provided (for example Camden Town), however this is for convenience to allow for when one escalator is out of use, I don’t believe it’s a requirement as such.
What will be an issue however is that if no up escalator service is available then it may be written into a station’s safety plan that it must be closed, simply because people aren’t deemed to be able to evacuate quickly enough via a stopped escalator (or stairs where provided). Such a policy is more likely the deeper a station is.
Modern stations require secondary escape, normally in the form of at least one dedicated escape stair, often two. Merseyrail seem to have sort of retrospectively ticked this box, albeit in a rather basic manner - for example Hamilton Square has the old Shore Road subway, Moorfields has the Old Hall St exit, and Lime Street has the running tunnels. I doubt any of these would be considered to meet the requirements of a new-build.