Would you need to know where the vehicle was actually manufactured for this? Could be different rules depending on whether most of the construction was in the UK or in the EU. So could be different rules depending on model and place of manufacture.
See here for the details on Preferential Origin. Now, I have yet to see a list of cars / models that meet that criteria to avoid customs, and keep in mind that on some UK manufactured/ assembled cars, changes in spec level can mean it comes from a different factory/country.