Hi,
Not sure that this is him but seems likely.
William Henry Threadwell born 1883, Stratford, West ham , Essex joined the Militia, Essex Reg 44 th and 56 th foot.
Attested 1 st August 1900
There seems to be a follow on record for a Henry Threadwell born West Ham in the British Army Pension Records.
It says he was posted 22.02.01 but not where to ,although he was in South Africa 25.04.02
Mother Mary, sisters Alice and Florrie (both married)
If you think this could be your William, he may have been out of the country at census time.
Best wishes
Cathy