"Closest" depends on which terminal.

I have been parking on Orchard Road in Belfont since 2015. The longest period I have parked for is 4 weeks. There are always free spaces and plenty which are not outside anyone's house. Many houses have driveways anyway. If there were issue with airport parkers I think the council would have introduced a permit system by now.

There has never been a problem with my car. Once it appears my car was moved for workmen to repave the road, then it was moved back, but that wasn't a problem.

If you have luggage you have to walk to Hatton Cross which takes up to 15 minutes - the tube is now free while the bus is paid. Without bulky luggage you could run across the A30 (you can walk on the central reservation) which leads out outside the Premier Inn, from where you can take a bus to T5, or walk further in to T4 for the tube/Crossrail to T2/3.

Other options are further along the A30 (https://www.google.com/maps/@51.4565...8192?entry=ttu) and around Stanwell, or Firs Drive and Park Lane to the northeast side of the airport.

Also if you have luggage and a travelling companion, you can drop off and pick up for free outside Hatton Cross station while you go to park - I find that easier than the free 30 minutes at Long Stay.