If some of the jet fuel comes from plastic waste instead of from the ground that might result in burning less fossil fuel, compared to the situation in which the plastic waste goes into landfill. However, as long as people are still burning coal to make electricity (which still happens in China, India, Australia, ....) I suspect that even less carbon would be released into the atmosphere if the plastic waste were burnt in a power station rather than turned into jet fuel.
No idea, really, but generating electricity by burning plastic might release less carbon than from burning coal because the plastic contains hydrogen as well as carbon. It depends, no doubt, on the type of plastic and the efficiency with which it can be burnt while avoiding the release of nasty chemicals.
It's not at all obvious to me what's best: landfill, burn, or recycle. But government policy should be based on a proper analysis of those options. I have the impression that government is mostly just compromising between what industry lobbyists are asking for, in order to maximise their profits, and what the ignorant public are asking for, in response to fashion and irrationality. Let's hope that somewhere some sensible civil servants are quietly doing the right sums.
