And ditto on a rammed solid P4 on the evening rush out of Brixton about two years ago. Police completely hopeless and CCTV apparently didn't show anything - I personally don't believe they even bothered to look.

I always make a point of zipping up my bag and keeping a firm hold on it when it's crowded. Possible other deterrent is to have a very large bag (fashionable these days) and fill it full of stuff, so no one - least of all you - can find your wallet in it!