Anything is within walking distance if you're willing to walk...
Anyway here's where it is:
https://maps.app.goo.gl/YZKPXp7LSjc5jPot5
Turn on satellite view and you can see it is on the edge of the long stay parking. I dunno if you're realistically going to save a lot of time unless your parking space is right at the back of the long stay car park. Worth it if it's free! I assume it's overpriced though.
From the map. It looks like the train is closer to the terminal :)
From London proper, it's best to take the train to the airports. If you're lucky you're flying out of London City Airport, but most of the time I'd go to Gatwick or Heathrow by train and it's a good way to get there.
Yes if you can get a train into a UK airport that's definitely the way, but even in the train-friendly UK that's often not feasible. For example Bristol airport has no railway. Heathrow is very inaccessible by train from the west. So there are unfortunately still plenty of reasons to drive to UK airports.
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