Unsecured lending rates - supernormal bank margins

The Bank of England (BoE) lowered its interest rate to just above zero in March 2009, over two years ago.  Since then banks have increased their lending rates to borrowers.  The actions of the BoE allow banks to borrow at historically low rates (as low as 0.1% - from many savers) and lend at over 20%.  Bingo!

Overdraft rates have not been this high at any time in the BoE record (since 1995). And remember that BoE rates in the period of the graph above were in the range 5-9%.  Bank margins on this business should be at record highs.  Is this what the BoE want?  To rebuild bank balance sheets at the expense of both savers and borrowers?