The IR35 intermediaries legislation has been in force for many years, but up until April 2017, if they were trading through an intermediary e.g. their own limited company, it was up to the GP locum to decide whether this legislation applied to them or not.
In most cases, where the locum did have a limited company, they would have considered that IR35 did not apply to them, with their company billing the practice for the full amount of their fee, and the locum then taking a low salary from their own private limited company, paying tax and NI, but paying themselves in larger share dividends.
Funnily enough, the government didn’t like that. So they’ve ended it by changing the rules for public sector companies, so it’s not up to the GP locum anymore, but down to the employer i.e. the GP practice, or similar employer, to determine if IR35 applies.