If that 20k is net of VAT, and you're working through your own limited company, in the
best possible case you could take home £17,767.20 of that £20,000 for an effective tax rate of 11.2%.
How? Start with the 20k. Pay yourself a 10k salary (tax free, pay £232.80 in national insurance). Pay 20% corporation tax on the remaining 10k (£2000). Pay the remainder to yourself as a dividend (which already has a tax credit applied and results in a 0% liability up to a certain limit).
Now, this does make the assumption this is either the first month of your tax year or the only income you make ;-) If you were to make 20k EVERY month, you would start to increase through brackets and pay more tax on the dividends.
By my calculation using ContractorUK's dividend tax calculator, at 20k every month and assuming no deductions at all, you'd pay around £53k on the total £240k for an effective tax rate of 22%, but I could be wrong. You would also be likely to pay more tax since you'd probably pay yourself a salary despite the loss of personal allowance in order to keep your state pension privileges.
Broadly speaking, direct taxation on income is generally lower than many situations in the US. It's the consumption taxes like VAT (20%) and fuel duty (the majority of the price of fuel) that bump things up a lot. I should also note that if you merely happen to be self employed rather than own your own company, your taxes are actually LOWER here than if you're employed - no equivalent to the "self employment tax" I've heard of in the US.
