Customs Duty is usually based on the customs value of goods and is usually calculated according to the value — ’ad valorem’ in Latin. This means that a tax will be charged by applying a rate (i.e. percentage) on the value of the goods. Most goods are taxed in this way, although sometimes different criteria are used such as weight, dimensions, quantity, etc. For instance, flour is taxed by weight and clocks are taxed by quantity.
For further information, please refer to GOV UK.