Starting from SE london as well, I prefer going via Woolwich ferry, Barking, Becontree, Romford to get out of London. You can take some suburban residential roads in Romford then quite quickly into the lanes heading north. It's not exactly scenic but I prefer it to going through london.
Yeah that's a good shout. I've even done the Gravesend>Tilbury ferry in the past. It's pretty but I must say Essex is my least favourite county for riding that's close to London.
This is the route I used, as @Thrasher says if you were to get the train to eg Romford or Brentwood and start there there'd be no love lost...
https://ridewithgps.com/routes/33315781
Hadn't realised it was in lost lanes but this thread has people nattering on about various local amenities/pubs if useful.
https://lostlanes.co.uk/23ee/