Unfortunately, the anti trade union legislation which the "New Labour" government failed to repeal means that refusing to cross a picket line constitutes industrial action.
Legally, if you are a member of another trade union which is not part of that strike, or a member of the same trade union but not in the dispute, then refusing to cross a picket line is unlawful industrial action because it would be classed as secondary action.
If your business faces industrial action - GOV.UK
It is a bastard, but there it is.
Unfortunately, the public sector has clamped down on this in recent years, and many have changed their policy to make clear that they will consider disciplinary action against anyone who is not officially part of the trade dispute refusing to cross the picket line.
An anomaly in the law, which is not relevant to your position, but might be to some of the cleaners, is that someone is not a member of any trade union, but is part of the dispute is protected as if they were a member of the union taking the action, so they can take industrial action. However, if they are members of a different union, even if they are on the same contract as the others, it is classed as secondary action.
This is something which the civil service spotted and has been exploiting in order to divide the unions, I reckon.
They are mostly all bastards.
HTH