From what I understand, you always get the electricity for 7p/kWh (in the form of a bill credit).
if you tell OVO you need it charged in 8 hours and it will take 8 hours, it will start charging immediately even if electricity is expensive for them at the moment – but you won’t pay any extra.
if you tell OVO you need it charged in 24 hours, they will hold the charge until electricity is cheaper for them, but will make sure the car is charged for when you specified.