Subject: Used books shop in London

while searching for used books shops in London, as I need to refill my quota of English books without spending a fortune, I ran into a Salvation Army Bookshop on Oxford Street. I can't of course hope any of you have been there, but I'd like to know if it could be something like a thrift shop in the USA (very good bargain prices) or it is a shop for Christian books only (I found it also on a list of Christian bookshops). Maybe an English Ziner could help? Thanks in advance and regards

Paolo Trieste, Italy