In London the banks are generally open:-
9.30 - 16.30 Monday - Friday.
9.30 - 12.00 Saturday.
Closed on Sunday.
Some branches offer extended opening hours, ie. opening at 8.30 and closing up to 17.00, Monday to Friday, and 16.00 on Saturday.
In the City of London the banks are open:-
9.30 - 15.00 Monday - Friday,
Closed on Saturday and Sunday.
The City of London is approximately the areas near Bank, Liverpool Street, Monument, Cannon Street, Mansion House, St. Paul's and Moorgate underground train stations.