I have no experience of PHEV’s but doing the math’s, I think charging from a 3 pin plug is the most cost effective solution. The difference between 3 and 4 1/2 hours is relatively small compared to the 9hrs vs 24 hours for a fully electric, when a fully charged battery is not an essential.
Obvious advice, but avoid the 69p kWh chargers, at current petrol prices its as cheap to fuel as it is charge.