I'm in the process of giving up working in East London because both the constant parking problem and also because of the traffic. The problem isn't only traffic wardens and actually finding a parking spot, yellow lines or not, it's getting tickets by camera.
Also, a trolley system is an absolute must. Yesterday I did a job and the nearest parking spot I could find was about 300 yds away.
Having said that, getting work in the areas I work in (Aldgate to Bow) seems to be fairly easy because there aren't many window cleaners because of the parking problems.