Probably a few other incentives in play.
Reputedly the IR has tried this on for years and been rebuffed. Their PAYE system is a system designed by employees on a payroll for employees on a payroll. Payrolls are what they understand.
Also the Paymaster General of the time, AKA Red Dawn, was from the left of the party. She would be unlikely to look favourably on workers who weren't unionised.