Who are you employed by?
This is easy. The agency is the corporation that hires you - not the end client. They should have to take ownership of your holiday pay and tax etc. If you have your own PLC, then it takes responsibility (as it should be doing already). I think this rule, if it is properly implemented, means that everybody will be an employee of some company.
This, although a generalisation, is not necessarily a bad thing. I like the idea of agencies having to take real care of their contractors rather than just putting on a beer fest once a year in December.