What about not undertaking their job at all?
I also don’t see how they can be ‘withholding revenue’, it isn’t as though they are taking it themselves.
You could well be right, and to be honest I'm only half-remembering posts I saw elsewhere in the thread. Having said that, striking is protected in law (otherwise not turning up would just be gross misconduct) and I would assume doing your job badly probably isn't. But I'm not a unionised worker so this is probably all nonsense, and I defer to those who know better.
Yes, the two main forms of 'protected' industrial action in the UK are 1) the complete withdrawal of labour - a strike, in other words, and 2) doing only the minimum required to fulfil the terms of employment, i.e. 'work to rule', which includes overtime bans and the like.
Employees on strike forfeit their pay for the time they're on strike, and have that time deducted from their duration of employment when things like statutory redundancy payments are calculated. On the other hand, employees working to rule are paid because they are doing exactly what their job requires them to do, even though the withdrawal of "above and beyond" working like overtime can still cause the company losses.
There is no middle ground - employees who turn up and claim their pay despite doing less than their contracts require, or who deliberately do their jobs badly, are subject to disciplinary proceedings and can be dismissed for misconduct.