Look here:
https://www.gov.uk/what-you-must-do-...payment-status
I know this is for the building industry, but I believe it also applies elsewhere.
If you're a self-employed contractor, you need to jump through some hoops before you can avoid having 20% deducted.
Personally, I have a limited company from which I issue invoices. These are paid gross, then I do a tax return including a CT600 (Corporation Tax). I usually combine this with the Companies House return, which makes the process relatively painless.