My criterion for what should be in or out of the state sector is simple. If it can or ought to be rationed by price, it should be privatised: if it cannot or ought not to be rationed by price, it should stay in the state sector (eg defence, police, fire, health and education). On this philosophy, there is nothing wrong with privatising the Post Office (providing it is required to deliver to any address within the UK for the same stamp).
I was once in a minority of one at a Liberal Assembly voting for the privatisation of water, and in a minority of three voting for the privatisation of electricity. I was also told in a Liberal Party Economic Affairs Panel meeting that I was in cloud-cuckoo land calling for the privatisation of British Rail.