Cosmos, Thanks for the additional information to clarify.
I really think your next step would be to order the marriage certificate, as you need to identify exactly who Thomas Brown was. This may help to find a death for him. Then you may know better when the family returned to England from India, and help understand when an informal adoption might have taken place.
Marriage certificate - Annie Ruth Raven to Thomas Brown
Jan-Mar 1903 Reading Berkshire
Vol 2c Page 589
I have looked for shipping records of their return travel to England between 1908 and 1911, but could see nothing. Others may be able to find something, or it could be that the family returned as part of a military group.
There are numerous possibilities Thomas left the army; he died in India or England; or Thomas and Annie separated we can only speculate. Accessing the full birth record for the son born in India in 1908 may give you a better understanding of the fathers position in terms of work.
Note there is a death of a Thomas William Brown (age 31) on 15 Nov 1909 in Lahore, Bengal India. There is also a death in 1923 India of another Thomas Brown (age 50).
I dont know if either is the right man, but you could follow up on them.