If you buy a vehicle you can offset a percentage of the vehicle cost as a capital allowance meaning you'll pay less tax on your earnings.
If it's classed as a commercial vehicle they give you a higher first year allowance which is 50% of the vehicle cost , a non comercial vehicle is 25%, both restricted to a maximum of £3000. Each year after that it is 25%.
So for a non commercial vehicle that cost £10,000.
In the first year your allowance is 25% of £10,000= £2,500
The vehicle is then valued at £7,500 for the second year.
Second year allowance is 25% of 7500 = £1,875
The vehicle is then valued at £5,625 for the third year.
So in your first year you made a gross profit in your business of say £30,000 you would take 2,500 off that aswell as other capital allowances and other expenses etc to get to your net profit which is what you are taxed on.
In your second year you take £1,875 off your gross profit etc.