I rarely ever carry more than £30 at any one time - mainly for a couple of pints in the pub and a pack of fags, which are just shame-inducing when you pay by card and they turn up on your bank statement!
This is generally enough to get out of most problems - but the main reason for not carrying more is the potential for theft. Pickpockets at mainline and tube stations, or worse street muggings on way home at night. If you haven't got it on you, they can't take it.
London, and I'm guessing many other big cities, are war zones for this kind of stuff - in this case not carrying too much cash becomes less of a choice and more an essential personal security measure.
Mate I have been mugged in a proper hardcore sense of the word but I had no money anyway so they got nothing and actually got collared and got sentenced. As for being pick pocketed, I have travelled a fair bit and keep my money in different pockets normally with a drop wallet. If all your money is on one card you loose it all in one go if who ever is robbing you gets it.
London is kids stuff. Carry a drop wallet and take precautions. And even that is a bit much in London.