As long as they have their own specialist equipment and maintain some of their own work then they can be self-employed. Another criteria is that they need to be invoicing more than two people.
Isn't there a 50% rule somewhere (I haven't read the link I provided)? I remember a few years back a kind member here who ran a large window cleaning company e-mailed me a 'work around' the rules where you could employ by making your wife start a window cleaning business on paper, then your 'employee/subcontractor' would invoice your company and your wife's company; or something along those lines.
I think the rule being is that if you provide anyone with more than 50% of their work, no matter whose tools they use or what the circumstances, then you will be viewed as the employer should anything go to court.
But he did stipulate that by using his method that should you ever end up in court you're on thin ice.
The above should be treated as no more than hearsay, because I am unsure of the full facts.