Brent Cross is an option. Right by M1 J1, with nearly 4000 free parking spaces. Disadvantage being that you need to get a bus (210 to Brent Cross, or 186 to Hendon Central) to access the tube network.
Aha, I see, the free car parking is the Brent Cross attraction!
It's quite possible to walk from the shopping centre to Brent Cross tube station (I've done it), but it's undoubtedly a somewhat awkward and 'pedestrian hostile' route.
Indeed, given this sub-optimal pedestrian provision, I'm not quite sure why on the shopping centre's opening in 1976 LT decided to rename the station from
Brent to
Brent Cross - if you're not in a car, you feel like a bit of an afterthought here!
(Re the free car parking at Brent Cross - there's nothing on
their website, but I would strongly assume that overnight parking is not permitted - furthermore it states the car park closes at 9.30pm, so there's at least the possibility of having your car stranded in there overnight... though I suspect there may be a way out that's open later than this.)