Most imports are valued using transaction value: the price you pay or owe the seller, with set adjustments. HMRC says it includes all payments that are a condition of sale.
Some costs must be added if they are not already in the price. Assists are one: tools, moulds, materials, engineering, design and tooling supplied by you or on your behalf for making the goods. Packing and container costs, and royalties or licence fees that are a condition of sale, must also be added.
Discounts must be agreed before the import entry is accepted. A discount given after import does not reduce the customs value.
Sources
A summary of the public sources above, checked when it was published. General information, not legal, tax or financial advice.