Without a doubt, the quickest airfield from Waterloo is Stapleford. Head over Southwark bridge, turn right and straight along Upper/Lower thames street, the highway and through The Link at Canary wharf, enjoy the veiw as you pass right over the threshold for 10 at City airport. Straight up the 406 and continue onto the M11, J5 and your almost there. Average traffic you should do it in 45mins. I have many times
Not the cheapest place though! If you have a license then continue up the M11 for 10 minutes to J7 and go to North Weald and see Simon Bowman who has 4PLanes at really good rates! Happy flying!