Different items can have different customs duty. Depends on the Customs Commodity Number (CCN), there is a list at
https://www.gov.uk/guidance/classifying-vehicles#classifying-vehicle-parts-and-accessories For example a dipstick is CCN 9026108900
Pop this into the search box in
https://www.trade-tariff.service.go...UR&day=10&month=10&year=2019Here we work down through the definition
90 Optical, photographic, cinematographic, measuring, checking, precision, medical or surgical instruments and apparatus; parts and accessories thereof
9026 Instruments and apparatus for measuring or checking the flow, level, pressure or other variables of liquids or gases (for example, flow meters, level gauges, manometers, heat meters), excluding instruments and apparatus of heading 9014, 9015, 9028 or 9032
902610 For measuring or checking the flow or level of liquids
9026108900 Other (not electronic) - Other (not a flow meter), which results in VAT at 20% and Duty at 0%
Whereas an ECU is CCN 85371098 and the VAT is 20% and Duty is 2.1%
The tricky bit is deciding on the CCN if it isn't obviously in the CCN list
Then Duty is payable on the Goods plus Freight (outside of EU) and Insurance - and then add VAT - also add the import agents fees
