Do you need car insurance if the car is not being used in the UK?

Yes and No!

To clarify, if the car hasn't been granted a SORN (Statutory Off Road Notice), which declares the vehicle off the roads and not in use, then insurance must be placed and remained on the vehicle even if you don't plan to ever use it. Once SORN'd, the vehicle is not viable to be insured, but driving it will be deemed illegal and the vehicle will be seized and crushed and you will incur penalties and court costs.