Southampton to London is £41 return for 160 mile round trip. Petrol alone will set you back half that. If there's a company willing to rent a car for 300 miles

Southampton to London is £41 return for 160 mile round trip. Petrol alone will set you back half that. If there's a company willing to rent a car for 300 miles for £20, they're making a loss, and that's before you get onto parking charges. Either way the train is far faster.