The move appears to be a response to lessons learned during the Waking Shark II security response exercise last November. The exercise put merchant banks and other institutions in the City at the wrong end of a simulated cyber-attacks and didn't involve retail banks.
According to sources who spoke to the Financial Times (behind paywall), the Bank of England's “ethical hackers” will attack 20 major banks and other financial institutions in the new round of cyber resiliency tests. Unnamed government-accredited penetration testing firms will be involved. The FT speculated that the Royal Bank of Scotland and the London Stock Exchange would participate, but there have been no confirmations