I was in London last week as well as in April, and I noticed that alot more men carry messenger bags than backpacks.
I could pick out the tourists easily by the way people dressed (Americans in backpacks, baseball caps, fanny packs, women in jeans, etc) and I always make sure I dont stand out as a tourist so I assimilate to their style by wearing skirts,dresses, Scarves, tights and flat soled boots rather than tennis shoes and jeans, etc...
Don't make yourself a target for theft!!