I think I've noticed signs at most motorway services that say a 2hour limit for parking. Those would be ideal spots for people to meet up park and continue in one car. Maybe the government should encourage these places to allow people to park for longer for reasonable price?

Update. I just looked up Lymm services on the M6, free for 2hrs, £16 for 24 hours... quite pricey for a weekend up to the Lakes (it would be cheaper to both drive). At least there's some posh areas around there.