Got fully comprehensive insurance but have scrapped my car, can I still drive other cars 3rd party under this policy?

In a word, no. The insurance you took out is for a car that you are the owner of and as it is in a scrap yard it is no longer yours.

Your car insurance policy is linked to the car that you have insured and if that car no longer exists, then the policy does not hold anymore. Therefore you will have to add another car to the policy for it to be active and then if you have the drive other cars' facility on the insurance, you'll be able to use it. Otherwise, you'll be driving without insurance.