Memberships look terribly expensive?
Can you go to a wall on your own? or do you always need a buddy? (who belays?)
Do most places charge for the use of a harness etc?
Membership generally only works out cheaper than paying for single sessions if you are going to go more than once a week.
As others have said, you can go bouldering on your own, most walls have an induction process where you get show around the wall and show the dos and don'ts - this looks like the Arch's version http://archclimbingwall.com/lessons/introduction/
The Arch in Bermondsey is one of the best bouldering walls in the country and is quite handy form Brixton.