Hi
There are censuses, civil registration and parish registers.
The 1911 census with the other family information about another brother William (nothing showing on censuses) leaves you with the possibility perhaps that a son may have been 'given up'?
Wiliam Hast Westhorp's birth wasn't in Poplar like the other three children, but Louisa Catherine was getting on by 1886 - she would have been about 45.
Births Jun 1886 Westhorp William Hast W. Ham 4a 80
Births Dec 1840 Westhorp Louisa Catherine Stepney 2 421
29th October 1878 The Standard
Marriages
Westhorp- Westhorp
Oct 26th at Buckhurst-hill Congregational Church by the Rev W H Charlesworth assisted by the Rev H J Bevis of Ramsgate, William Hast Westhorp to Louisa Catherine, eldest daughter of T. Westhorp, Esq. of Loughton Essex.
1871 census RG10 16?8 folio 49
Holmehurst Manor Road Loughton Essex
Theopolis Westhorp 57 Head Married Ship owner & ship chandler patent oakum manufactory 164 men and 20 boys Harwich Essex
Rebecca Westhorp 56 Wife Married Poplar Middlesex
Louisa Westhorp 30 Daughter Poplar Middlesex
Alice Westhorp 16 Daughter Poplar Middlesex
Eliza Westhorp 77 Sister Unmarried Harwich Essex
plus 2 visitors and 3 servants
First World War army officers records are not online. This may be William's records at The National Archives
WO 339/66267 WESTHORP W [1914-1922]
William Hast Westhorp senior left a will as did his wife. Both wills could be sent for and of course there is the birth certificate which would settle it one way or the other.
Regards
Valda