Not to go into too much detail, but if you start trading before April 09 you will be paying tax by next January and plus, if your income is above a certain rate you will also be making a payment on account for the following years accounts ( about half of your current tax and NI liability along with your first payment), and also first year carry over if applicable. This is all based on if you make a profit in the first year, which you plan to.
Also, I am not sure what your current income is, but remember that once you earn over circa £35k you will be paying 40% tax.