I'd come to a similar conclusion. That PHEV is a practical way for me (single car household, most trips under 20 miles a day) to greatly reduce the petrol I burn. Even charging at home the saving would be significant. I think it's the best option to reduce emissions too. My Mk1 Jazz is 134 (g/km), at best an ICE car might get that down to about 100. It's difficult to say quite what the emissions in real life would be but my feeling is less than 80, so long as I did actually charge it most of the time (from renewables).
As you imply the initial cost is the obstacle. Looking around the cheapest secondhand PHEV is a Vauxhall Ampera. Though I don't think it's suitable for me it's an interesting car. The product of GM throwing a lot of money at the problem. Basically the technology is the same as the Mk4 Jazz - the wheels are driven by an electric motor which is charged from a petrol engine. With the difference that you can also charge the batteries from a plug. It wasn't a commercial success, mostly because it was so expensive to buy. Seems to me like good value, now that someone else has paid the depreciation.
I suppose the batteries in a Mk4 Jazz are too small to make it worth adding a plug in charger.