Edward Green, Crockett & Jones and Gaziano & Girling use "E" for medium and "F" for wide. Though I do find G&G lasts quite narrow. C&J also do an extra-wide "G".
J Fitzpatrick also calls his (only) medium width "E".
John Lobb also uses "E" for medium, but "EE" for wide.
Church's, Cheaney, Barker, Loake and Grenson use "F" for medium and "G" for wide. Church's have an extra-wide "H".
Tricker's use numbers, just to mess with you. If I remember rightly, 5 is...