If it's value is under £36 no duty for GIFTS (a gift has to sent from a private address to a private address, NOT from a business to a private address)
If it's value is under £18 for anything else
If the duty is less than £7 they don't bother collecting it, different rates of duty apply to different classes of items so the actual value of goods that escapes duty varies.
Car spares duty is around 4%.
Duty on a whole car is 10%

Vat is also charged on top of the duty, based on goods value, shipping cost and Import duty value.

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