Ok, sorry about my rant! Sore point, I hate the use of the word subbie and sub contractor when used in the wrong context.
He would be taking advantage because he's getting labour without the commitment. Of course you have no commitment to him either so you can go and work else where whenever you like, but the law is there to protect you not him. As an employer he has a duty to take care of your tax through paye, making you self employed means that is now your responsibilty, so he makes the profit from your work and without the commitment. Any agreement regarding amounts of work etc would not be binding so at any time he can say sorry no work next week and your stuffed.
Will you be providing your own tools, ppe etc?
I'm not anti self employed people. I soley use self employed people, for the same reason as your boss wants to, but I'm just trying to offer some help/advice. My main worker also works for three other firms so I can't rely on him being available 5 days a week if I suddenly need him. That is how self employed should work IMO. If I wanted commitment of 5 days a week then I would have to employ someone full time PAye and all. You can't have your cake and eat it as an employer.
Years ago I was in the same position as you. Employed at about £27/day. My then boss said he couldn't keep me full time so I should go self employed at £35/ day. I agreed as I had no choice but I immediatly looked for other work. On my last day of employment I told him I wouldn't be in the following monday as a self employed person as I was starting full time with a bigger company. He went mental, but I moved on a did much better with the new firm. They paid for my tickets etc and two years later I was ready to move on again.
If you have no choice then accept the sefl employed option but just bare inmind that you are doing him the favor. Pay should be double what you are on now to make it worth your while. Effectivley the holiday pay that is not going to get paid needs to be included in your day rate.