I’m afraid my day’s of flying are over but my grandson often flies from the London airports and what he’s been doing is parking in a local Tesco superstore car park. The one he uses for Heathrow is about half a mile south east of the airport from memory. Cost are very reasonable and noticed that my local Sainsbury’s supermarket do the same here in Bristol where they put a sign up in their car park with instruction to pay though that said I think he pays online before going.
Not suitable for the disabled as it involves often a mile or so walking or taxi’s but even taking that on board its still a lot cheaper than airport parking.