In reply to Ross.Cooper:
whereabouts in Essex?
Essex uni in colchester has a small wall but not sure about access now.
Couple just over the border in Suffolk, although not sure on details.
Chelmsford apparently has a wall although dont think there is much to it.
There is Harlow as well.
Fairlop boulders for some outside stuff (artifical).
Chances are best to go into London, depending where you are Mile End could be pretty easy and quick to get to being on that side on London.