What is the best and easiest way to go from Sutton to Barking??
Train then underground 
a217 out to m25 then either north or south bound depending on the time through tunnel would i think be quicker
A217 onto M25 J8 then through the dartford tolls, Off J31 down the A13 and bob’s your uncle.
If it’s 3am on a sunday morning, you might like to try going through London directly…