£3.00 for 20 miles works out at 15 pence per mile.
Unleaded petrol @ £1.40 litre at 40 mpg = 15.9 pence mile.
Most modern hybrids achieve around 50 mpg = 12.7 pence per mile.
You would be better of using your PHEV as a hybrid as Glos Guy suggests and save yourself all the agro.
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.