R1Geezer
It would be difficult to implement, because it cannot be done unilaterally, and banks would have to spin off their (very profitable...normally) investment business from it's traditional retail arm. From what I remember from my days working for one of the high street banks, it was the commercial and investment arms that made the big bucks...and coupled to that, you'd need to consider the investment portfolios of charities, mortgage brokerages and pensions administrators who invest in products under the control of investment firms.
If we floated them off, and they crashed, the ripples spread much further than the City Of London.