Stopover en route to Dover - Layby on the A25, between Limpsfield & Westerham. Leave the M25 at J6 and follow the A25 for about 10 minutes max going steady. The entrance to layby is just before a large pub called The Grasshopper with huge height barriered car park. (Layby slopes and is only levelish at the bottom). To continue to Dover you need to go on to Junction 5 of the M25 but there is no access to the M26 from that junction. You have to go to Swanley to get onto the M20. Otherwise it's back to Junction 6 for the M26/M20 less miles route. A matter of personal choice, although, you could use the A25 to Wrotham and join the M20 there.
Free camping near Tower Bridge
There is a lorry park close to central London, just by Tower Bridge, parking for 24 hours is £20. It is frequently used by motorhomes and overnighting is allowed. Security is very good. Coming south over Tower Bridge, down the slope off the bridge will be first right and right again as is on river side with some of the park almost under the S-West corner of bridge. Phone No. is 020 7378 1147 (Tooley St.)
Free camping in Lincoln's Inn Fields
Lincoln's Inn Fields, is about 150 yards from Holborne tube station & 500yards from Oxford St. or the Thames. The "fields" is a square just off Kingsway and has a large number of Company HQ's spaced so there is a single metered parking space between their gates, Lots of car parking plus room for about ten 25 foot motorhomes. Maximum stay is 2 hours between 8.30am & 6.30pm & free on Sundays & bank holidays. Cost is £2.40 per hour.