They are "UK based" in the sense that any resources in the UK sector are by International Law, UK Crown assets. They have 'licenses' to extract awared by UK Government. But in times of National Emergency/War a Nation can step in and alter any such licenses. After the crisis is passed, things could then revert. Doesn't matter if the offshore companies are 'UK' ones or not.
The UK Govt/we have been getting that benefit one way or another. According to Wiki £330 BILLION has been paid in production tax since 1970.
It has been spent somewhere, unlike Norway which has saved it and created the worlds largest Sovereign Wealth Fund.