How much did a phone call cost per minute from UK to Australia in the 70's and 80's ?

The modern experience is not perfect but audio is usually far better than the 70's and 80's, video is often an option, and in general the experience is orders of magnitude cheaper.

PCMU and PCMA voice frames are 20ms or 40ms, and no one is running with no jitter buffer, so your 30ms number doesn't make sense.

Even circuit switched networks are not often below 30ms, to hit that you'd need to make a local phone call on a fully analog circuit.