There is a little thing called IR35 which stops people doing that I'm afraid.
And when you get caught.... and you will.... the Tax Man will no only want your employees contributions from you but he will also want EMPLOYERS contributions....
So any savings you would have made will be wiped out.
And remember... the tax man can go back 7 years.... so you could bimble along quite happily thinking you are in the clear and then WHAM !! Huge Bill with 14 days to pay.
To work for an Agency in a tax efficient manner you would have to set up a Limited Company and a Limited Company is an entity in its own right, that can sue and be sued. You may be the owner and technically the employer, but you will also be an employee of that company... Hence you would get hit for the employees and employers tax & NI contributions.
A friend of mine has just had a visit from the IR35 team to see if he is inside or outside the scope of IR35
He's ok BUT they gave a chilling warning that you are NOT safe working for an 'Umbrella' company.
So just pay your tax and be happy with your lot, you only pay tax when you are earning money....