The article states "a fifth of Londoners having had their bank details stolen" which seems to imply of London as a whole, not just those on the register. With London's population at around 7m, that seems like a truly ridiculous figure. That said, it makes sense that London has significantly more than elsewhere, the capital city generally does, especially one that is as dominant in terms of population as elsewhere.
But you don't have to be dim to have your bank account tapped into. It happened to me despite my shredding all my unwanted mail before throwing it away, despite paranoia over who gets my bank details, and despite using careful, well thought-out passwords. First sign I got was checking online banking and finding that my overdraft had been maxed out. Not an experience I recommend, but to be honest you're powerless - your details are as secure as the operative in the call centre who gets access to them daily wants them to be, whether they be at your bank (every detail under the sun), mobile phone provider (name, address, credit card and/or bank account), cable television company, etc, etc, etc. It's a long human chain, one that relies on trust and integrity, and one that is, sadly, always going to be exploited.