the UK test for whether you're a contractor is ... how many different people do you work for?
HMRC would love the test to be that simple, but it clearly isn't. To suggest that a company must abandon a customer who provides regular business simply to prove its independence is an unreasonable restraint of trade. There probably exist companies with one customer and numerous employees - how do you categorise them?
However much they may trumpet their occasional victories, the truth is that HMRC's record in proving that contractors are really employees is pitiful.
I think I'll post anonymously, though. Just in case.