N & NW Campsites near mainline stations to London...

[Barry Chuckle]

Hi I'm after some guidance, I'm off to London later in the year and I've got to get close to the centre.

I appreciate the where to stay in London question has been asked a few times on this forum. And I'm aware of the the very few campsites around London, Crystal Palace, Lee Valley, Theobolds, Wyatts etc. But I'm looking for something a little further out either north or northwest of London to stay in the campervan as I'll be traveling from the Northwest. Ideally I'd like something very close to a mainline station where I can be in central London in under 40 mins so probably within a 50 or so mile radius.
I've stayed in some of the area's on the outskirts of London near the end of the Tube lines like Wyatts Covert & Theobolds Park and ironically it's quicker to get into Central London from places like Milton Keynes than the aforementioned by the time you've got to a tube station and made the journey in. Also further out places tend to have a nicer rural feel.

Any help would be greatly appreciated!