Is policyholder responsible for named drivers actions

If the named driver was driving whilst drunk, he is the one who's going to be prosecuted. The policyholder has nothing to do with that.

Loss of NCD only happens if you make a claim with your insurer and you're at fault, in which case the car insurance company will not be able to recover its costs and therefore reduces your no claims entitlement.

The main problem with having additional drivers on a car insurance policy is that they don't "really" get NCD as such and it's only the policyholder who earns the no claims at the end of the year. So that means that if a named driver were to make a claim, it is the policyholder who will lose out on the NCB.