Tricky-- due to german forgeries all notes over �5 were withdrawn in 1945, as you will know the �10 & �5 notes were intro in 1759 &1793 , this leaves �20, were the �50 around then, i dont think so
sorry to make it three choices but I am sure I have read that the Bank of England introduced a few One thousand pound notes which were withdrawn a long time after - will try and check but would be pretty confident.
the following notes were introduced in 1833 and were withdrawn from use in 1945 : �10, �20, �50, �100, �500 and �1000. Before 1833 notes were issued but they were only promisary notes used in financial trading