The Attwood family in 1851:
HO107. P. 1512. Fol. 392 Pg. 15
Stanhope Street, St Clement Dane, London
ATTWOOD
Henry Head M 31 Police Constable B. Wilts. Salisbury
Sarah Ann Wife 32 B. Berks, Wokingham
Henry Son 7 B. Middx. St Martins
Alfred Son 5 B. Middx. St Giles
Eliza Ann Dau 2 B. Middx. St Martin
and in 1861:
RG9. P. 180 Fol. 62 Pg. 16
10 Star Court, St Clement Danes
ATTWOOD
Henry Head M 40 Labourer B. Wilts. Salisbury
Sarah Ann Wife M 32 B. Berks. Wokingham
Henry Son 17 News Agent B. Middx. St Martins
Eliza Dau 13 B. ~ ~
Sarah Ann Dau 2 B. Middx. St Clement Danes
This seems a possible for their marriage:
Q3. 1843 Shoreditch District Vol 2. Pg. 369
Henry Attwood and Sarah Ann Bedford on the page
ON the IGI there is a marriage Henry Attwood and Sarah Ann Bedford
7 September 1843 St. Leonards, Shoreditch
William George Attwood birth reg. Wycombe district Q2. 1864 Vol.3a Pg. 430
William's birth certificate will confirm Sarah's maiden name and thus the marriage (or not ! )
In 1871 Eliza seems to have a suname Etherington, and to be a step-daughter; I cannot explain this

In 1881 Sarah is a widow.
Death reg. for Henry Attwood B. 1822
Wycombe district Q3. 1879 Vol. 3a. Pg. 270
whilst looking for that I also noticed :
Henry Attwood b. 1844 died Wycombe district Q2. 1873 Vol. 3a. Pg. 383
This is possibly William's eldest brother ?