After those 3 days, I plan to rent a car (not sure if there would be a better plate to get it in London, plan to go by the website autoeurope). Will then travel to Stonehenge, Oxford, Liverpool, Edinburgh, Iverness, .... and finally make our way back by Yorkshire and the east coast.
So I don't plan to go back to leave the car in London, I would like to go straight to Heathrow and leave the car there.
Is there a problem with this plan? Will there be extra cost because it's not the same return location?