If you are thinking of casting off from the shores of being permanently employed and heading into the potentially stormy, but nonetheless lucrative, waters of contracting or freelancing, then you need to consider how you will set up your new business so as to minimize your long term tax liabilities, while maximizing your income and making it simple for your clients to deal with you. Your choices are to set-up a limited company or to operate under what is called an tax umbrella company. Setting up a limited company, in which you are the director and only shareholder, is the most tax efficient way of working, and means that you can claim back a wider range of expenses, such as equipment and software costs. The downside to setting up a limited company is the extra work involved. There is a lot more paperwork to fill in, and you will need to hire a lawyer and an account to look after your affairs, which can make the process quite costly.
So, if you want to focus solely on your work, and to bring in as much income as possible, then using a tax umbrella company is the better option. What they do is take care of your invoicing, payments and tax contributions. They do this in the same way as your previous employers did. They literally follow exactly the same processes. Technically, you are employed by them, but that is only for tax purposes, and not in a practical sense. Obviously, they have no control over you, and cannot tell you what to do or how many hours to work. The tax umbrella company will pay you weekly, fortnightly or monthly (whichever you agree on when signing a contract with them), and the funds will show up in your chosen bank account, minus your tax contributions and the umbrella company's fees. The umbrella company should only take a set fee (again agreed on before you sign a contract), and not a percentage of your earnings.