Goods that combine components from more than one trading bloc are subject to the Rules Of Origin procedure. Goods made in the EU are zero-rated on arrival in the UK, while the status of duty payable on third country components or ingredients used in EU goods is determined by applying Rules Of Origin. These establish whether or not the third country component has been transformed sufficiently for it to be considered an integral part of a new product. If it is a fellow traveller in a blended product, for instance, it is liable for duty.
The first target is to ascertain whether or not the component concerned has been absorbed into the finished EU product. If so, it can usually be covered by the duty payable on the finished product. If, on the other hand, it can be recovered from or identified within the EU product, the third country component may be liable for third country duty pro rata. The key marker is whether or not the third country component qualifies for a change of customs code. This will be decided by the UK customs staff on a case by case basis.
In the case of third country extra virgin olive oil (1509 2000), it is considered a fellow traveller in a blended product. This currently stands at 104 gbp per 100kg, according to the UK government online tariff service, https://www.trade-tariff.service.gov.uk/subheadings/1509200000-80 (as of check made on May 29).