A few points to add:
1. I had a CVT Mk 3 Jazz in 2015-16 and averaged 57.8 mpg (fuel into tank) over 5300 miles. This was mainly longer journeys at 50 to 60 mpg. A visit to and around Scotland averaged 62.8 mpg over nearly 1500 miles. However, it was a challenge to keep the vehicle running in the efficient Atkinson cycle mode as there was a tendency to substantially increase the revs when only a little more power was asked for (this behaviour was one reason for trading the vehicle in for my HR-V). I can well understand why the WLTP test hit the mpg claims because of the vehicle's tendency to go into a less efficient operating mode at each change in road conditions. I was under the imprssion that Honda tweaked the behaviour when the Mk 3 was refreshed but maybe I'm wrong.
2. I agree that the CVT is less efficient than a conventional mechanical gearbox. Usually this is offset by the software being able to keep the engine running in its most efficient operating range. However, a careful driver who knows what this operating range is can operate a manual gearbox to give the optimum mpg (eg the Land's End - John O-Groats trip involved someone from the factory driving a manual gearbox Mk. 3).
3. I would hope that Honda would use a battery pack larger than 1 kWh in the Mk 4 Jazz. Otherwise the little battery will have a hard life and the hybrid system will struggle to give much benefit in other than simple stop-start-accelerate-brake urban driving conditions.
4. Let's not overestimate the weight penalty carried by a PHEV. Assuming 40 miles battery range at 4 miles/kWh indicates the need for a 10kWh battery. For guidance, the 13kWh Tesla Powerwall domestic battery weighs about 130 kg. However, an automative battery pack will need better thermal and power management (the Powerwall is designed for a sustained 5kW output).
5. Visit Gridwatch
https://gridwatch.co.uk/ to see how wind power contributes to the UK generating capacity. I've seen it peak at around 12 GW. However, it is an unreliable source as shown by the attached graphic which I captured on Christmas day (the wind is light blue). Unless there's investment in some massive (multi-GW x multi-day) storage, I think we need to bite the bullet and invest in more nuclear if we are going to have a reliable source of electricity.
6. The sensible solution is to cut back on travel. Closer proximity of homes and work places would be a good start plus road charging to discourage Amazon and the like from moving truckloads of stuff up and down the country.