Strange cant find any date of death for Annie or her husband Thomas or son THomas Henry William Brown born in India
No idea who the third child was.
It is possible that the first born child Florence was also adopted out, and had a name change.
This may have also been the case with the son Thomas Henry William born in India.
If you purchase Florence's birth registration, it would give you her exact birth date, and this might help you find her in the 1939 register.
I would look in the 1921 census for Thomas born abt 1908 India. his place of birth might make it easier to find, if he is going by the same name.