But why would you drive? You can reach LGW by train from a wide area of southern England.
As I've said before, LHR has the airlines over a barrel. They can't afford to lose slots from lack of use because in normal times they are so valuable. (In the final days of British Midland their LHR slots were their main asset) So they move everything to LHR and rely on being able to get back slots at LGW when they need them.