When you work as self-employed in the UK you are entitled to a number of allowances designed to take tax off work-related expenses. To a reasonable amount, of course.
If you use your own vehicle for work, you can claim tax free 40p/mile for the first 10,000 miles. If you relocated because your employer wanted you to, you can claim up to £8,000 of moving expenses tax free.
If you are married, this allows you to shift assets and avoid higher tax brackets. Sometimes this may result in saving as much as 40% of your income.
Whether you have eligible expenses to claim or not, you will still be required to file a self assessment tax return each year.