Right, but 'self-employed' people who are constrained by contract to working for a single 'client' [employer] at that employer's effective beck & call aren't actually contractors at all, they are disguised employees. Having a pool of contractors you use on an ad-hoc basis for surge capacity & to cover full time staff out sick etc is one one thing, but of course unless they are also allowed to go and 'do a bit' for the competition when not required by you, that's a zero hours contract. Which again is a source of concern to people trying to enforce employment law.