standard VAT is 17.5% - if you want to find out how much vat you're paying on what you've bought, take the item price and divide it by 100, then times that answer by 17.5 - you'll then get what tax you were charged. dont forget, theres no tax on some fresh food products.
With VAT, the price is 117.5% of the no VAT price of 100%, which in turn is 0.85106 of 117.5%. Multiply the 117.5% price by 0.8511 and you get the non-VAT price.