Was Charles shown as a gardener on the 1928 marriage cert?
If so - there is a 1911 entry for a John Pearce ages 6 b Bucks - parents Charles & Frances. Age fits with 1939 birthdate
1911
Charles Pearce 37 Head West Wycombe
Frances Pearce 32 Wife Liverpool
Charles Pearce 10 Son
Thomas Pearce 8 Son
John Pearce 6 Son
Ellen Pearce 0 Daughter
All others born High Wycombe
Could this be their birth entries
PEARCE, CHARLES HENRY mmn HOPPER
GRO Reference: 1900 S Quarter in WYCOMBE Volume 03A Page 834
PEARCE, THOMAS mmn HOPPER
GRO Reference: 1903 M Quarter in WYCOMBE Volume 03A Page 880
PEARCE, JOHN mmn HOPPER
GRO Reference: 1904 D Quarter in WYCOMBE Volume 03A Page 390
PEARCE, ELLEN LOUISA mmn HOPPER
GRO Reference: 1910 J Quarter in WYCOMBE Volume 03A Page 977
There is this marriage
Charles Edward Pearce Age 25 occ french polisher
Marriage 7 Aug 1899 Liverpool, Holy Trinity, Lancashire
Father Henry Pearce
Spouse Frances Mary Hopper