The UK tax boundaries are as follows for the 07-08 financial year.

The first £5,225 of your earnings (that's money you earn in ANY way, whether it be selling lemonade or through adsense revenues) is tax exempt. Following that, the next £2,150 of your earnings for that year are taxed at 10%. The next tax band is for anything above that extra £2,151 - £33,300 at 22% tax. Anything above £33k on top of that original £5,225 and £2,151 is taxed at 40%.

On top of that, you have to pay your National Insurance tax.

Hope that helps UK people understand