According to the UK HMRC:
Difference between employment agencies and businesses
Employment agencies
Employment agencies find work for work-seekers who are employed and paid by employers. This is often called ‘permanent employment’ because once the worker has been taken on, they’re an employee of the company they’re working for. However, different rules apply to entertainment and modelling.
Employment businesses
Employment businesses engages a work-seeker under a contract who then works under the supervision of someone else. This is normally called ‘temporary agency work’ or ‘temping’.
Workers under these arrangements are paid by the business instead of the company they’re supplied to.
When a business does both it has to follow the rules for both employment agencies and employment businesses.
In short, the " workers" should be entitled to the same conditions as " employed" workers.. let's see how this one pans out.. It makes sense as now the agency "fees" become internal wooden dollars..