Is the William you refer to Edgar William ?

Name: Edgar William Trigg
Date of Registration: Jul-Aug-Sep 1908
Registration district: Westbury
Inferred County: Wiltshire
Volume: 5a
Page: 271

1911
TRIGG THOMAS RICHARD M 1886 25 Westbury on Severn Gloucestershire
TRIGG GERTRUDE F 1888 23 Westbury on Severn Gloucestershire
TRIGG EDGAR WILLIAM M 1909 2 Westbury on Severn Gloucestershire
TRIGG THOMAS ALBERT M 1907 4 Westbury on Severn Gloucestershire
TRIGG GERTRUDE ALICE F 1910 4 MONTHS Westbury on Severn Gloucestershire

in transcriptions
1913 TRIGG Ernest Frank Thomas Richard Gertrude Mitcheldean Gloucestershire [not stated] Forest of Dean Bible Christian
1916 TRIGG Ivor Verdun Thomas Rich [ard] Gertrude East Dean Gloucester [not stated] Forest of Dean Bible Christian
1919 TRIGG Donald Thomas Richard Gertie Mitcheldean (Plump) Gloucester Forest of Dean Bible Christian

possibles in BMD
Marjorie F Trigg Jan-Feb-Mar 1926 Westbury S Shropshire
Maurice H Trigg Jul-Aug-Sep 1928 Westbury S Shropshire

Name: Edgar W Trigg
Birth Date: abt 1909
Date of Registration: Jan-Feb-Mar 1957
Age at Death: 48
Registration district: Bristol
Inferred County: Gloucestershire
Volume: 7b
Page: 221