Here are all the middle names (excepting surnames used as middle names) used for Leo and Leon found on the 1911 England and Wales census and cross-referenced with the CRB.
Leo Adolf
Leo Adolphus
Leo Albert
Leo Alexander
Leo Alfred
Leo Aloysius
Leo Alphonsus
Leo Ambrose
Leo Amos
Leo Andrew
Leo Anthony
Leo Archibald
Leo Arnold
Leo Arthur
Leo Athenasius
Leo Augustine
Leo Augustus
Leo Austin
Leo Basil
Leo Benedict
Leo Bernard
Leo Bertram
Leo Bertrand
Leo Byron
Leo Camillus
Leo Casimir
Leo Cecil
Leo Cecil Clarence
Leo Cecil Vernon
Leo Charles
Leo Charles Henry
Leo Charlie
Leo Chester
Leo Christie
Leo Christopher
Leo Clarence
Leo Claude
Leo Clement
Leo Colin
Leo Corindon Samuel
Leo Cornelius
Leo Cosmo
Leo Cuthbert
Leo Cyril
Leo Cyril Edward
Leo Damian
Leo Daniel
Leo David
Leo Dennis
Leo Desmond
Leo Dominic
Leo Donald
Leo Douglas
Leo Dunstan
Leo Edgar
Leo Edmund
Leo Edward
Leo Edwin
Leo Eric
Leo Eric Robert
Leo Ernest
Leo Ethelbert Cyril
Leo Eugene
Leo Eustace
Leo Ezekiel
Leo Felix
Leo Ferdinand
Leo Francis
Leo Frank
Leo Frederick
Leo Frederick Anthony
Leo Frederick Austin
Leo Frederick James
Leo Frederick Percival
Leo Gabriel John
Leo Geoffrey
Leo George
Leo Gerald
Leo Gerard
Leo Gilbert
Leo Godfrey
Leo Godwin
Leo Gordon
Leo Gordon Henry Thomas
Leo Gregory
Leo Harold
Leo Harry
Leo Harvey
Leo Hector
Leo Henry
Leo Henry Horace
Leo Herbert
Leo Hilarion Haydn
Leo Horace Alexander Thomas
Leo Horatio
Leo Howard
Leo Hugh
Leo Ignatius
Leo Isidor
Leo Jacob
Leo Jacobus
Leo James
Leo John
Leo John Alfonso
Leo John James
Leo Joseph
Leo Joseph Patrick
Leo Jules
Leo Julian
Leo Kenneth
Leo Kevin
Leo Lawrence
Leo Leonard
Leo Leslie
Leo Louis
Leo Lucas
Leo Luke
Leo Mark
Leo Marmaduke
Leo Martin
Leo Matthew
Leo Michael
Leo Miles
Leo Morris
Leo Nephi
Leo Nicholas
Leo Noel
Leo Norman
Leo Oscar
Leo Oswald
Leo Otto
Leo Owen
Leo Patrick
Leo Paul
Leo Paulinus
Leo Percival
Leo Percy
Leo Peter
Leo Philip
Leo Raymond
Leo Reginald
Leo Reuben
Leo Richard
Leo Robert
Leo Roy George
Leo Rupert
Leo Samuel
Leo Scott
Leo Sebastian
Leo Sidney
Leo Stanislaus
Leo Stanley
Leo Stephen
Leo Stewart
Leo Stuart
Leo Sylvester
Leo Terence
Leo Theophilus
Leo Thomas
Leo Trevor
Leo Urban
Leo Valentine
Leo Victor
Leo Vincent
Leo Vivian
Leo Walter
Leo Walter Victor
Leo Wilfred
Leo William Claude
Leo William George
Leo William Joseph
Leo William Nicholas
Leo William Rolf
Leon Abraham
Leon Adonis
Leon Albert
Leon Albert Edward
Leon Alec
Leon Alexander
Leon Alfred
Leon Alfred John
Leon Alphonse
Leon Amedee
Leon Andrew
Leon Antonin
Leon Arthur
Leon Aubrey
Leon Augustus
Leon Augustus Stanley
Leon Benjamin
Leon Bernard
Leon Bertram
Leon Bertrand
Leon Cecil
Leon Charles
Leon Charles Augustus
Leon Charles Ernest
Leon Charles Henry
Leon Christopher
Leon Claud
Leon Clement
Leon Clifford
Leon Clyde
Leon Cornelius
Leon Cyril
Leon Daniel
Leon David
Leon Denis
Leon Douglas
Leon Edgar
Leon Edgar Joseph
Leon Edmond
Leon Edward
Leon Edwin
Leon Elliott
Leon Emanuel
Leon Emile
Leon Ernest
Leon Ethelbert Dudley
Leon Eugene
Leon Ferdinand
Leon Francis
Leon Francis Augustus
Leon Frank
Leon Frederick
Leon Gabriel
Leon George
Leon George Harold
Leon George Valentine
Leon Gilbert
Leon Godfrey
Leon Gordon
Leon Gregory
Leon Guy
Leon Harold
Leon Harry
Leon Henri
Leon Henry
Leon Henry Guy
Leon Henry James
Leon Howard
Leon Hubert
Leon Hugh
Leon Hyman Edward
Leon Ira
Leon Isaac
Leon Israel
Leon Jack
Leon James
Leon John
Leon Joseph
Leon Joseph Ismael
Leon Julius Bernhard
Leon Karl
Leon Leslie
Leon Lordan
Leon Louis
Leon Marcellino
Leon Marcus
Leon Maurice
Leon Miles
Leon Montague
Leon Montague Augustus
Leon Nathan
Leon Norman
Leon Oscar
Leon Owen
Leon Paul
Leon Percy
Leon Peter
Leon Pierre
Leon Ralph
Leon Rees
Leon Reginald
Leon Richard
Leon Robert
Leon Roscoe George
Leon Rudolph
Leon Russell
Leon Samuel
Leon Saul
Leon Sidney
Leon Solomon
Leon Stanley
Leon Stanley Trevor
Leon Stuart
Leon Sylvain
Leon Thomas
Leon Valentine
Leon Vernon
Leon Victor
Leon Victor Eugene
Leon Vincent
Leon Vivian James
Leon Walter
Leon Wilfred
Leon William
Leon William Henry
