I also had my wallet lifted out of my handbag on a heaving P4 setting of from Brixton just over a year ago. I felt the jostle, but only checked my bag, once I could actually move. Exactly as in the other cases related above, the thief got off at the next station. And the police - they didn't give a monkey's!!
There wasn't much money in the wallet - less than ?20, but the huge inconvenience of replacing all the cards (inc. loyalty cards) and the loss of all the receipts etc. and other bits of paper was a right pain! Also I was fond of my wallet and couldn't find a new one I liked as much.
I now carry a rucksack with the wallet at the bottom, so anyone wanting to pinch it would have to have a good old furtle around. So much better than at the top of an unzipped handbag! The trick is to make yourself into a less attractive target than the next person. These lowlives only go for easy pickings.