This was the official rate of interest charged by the Bank of England in its role as lender in the last resort. Since 1972 the term bank rate has been discontinued and has been replaced by the term minimum lending rate (M.L.R.).The principal effects remain much thc same, though the rates charged by the commercial banks to their customers are now not directly tied to thc rate charged by the Bank of England. The idea behind this change was to foster competitive rates in the genral money market but for the major commercial banks, movements in rates charged tend to remain concerted.
The historical list of bank rates can be found here.
Reference: Oxford Press Dictonary of Economics, 3rd edt.