You need a simple division. For example:
Suppose you have a VAT component of £14.90. What was the price Net?
14.9 * 100 / 17.5
1490 / 17.5 = 85.14
If the Vat was £14.90, the price before VAT was £85.14
In the UK there are three rates.
Standard rate: Most goods and services 17.5 per cent.
Reduced rate: domestic fuel or power 5 per cent.
Zero rate: food books 0
The value added should be the difference between what you paid for the input and what is paid for our output. Only on this difference do you pay the VAT.