Did you notice that that Israel Brown, was noted on those Military records as
NOT a British Subject

and
was Russian

and
was born in Poland

and
was married in a synagogue in England

and
the names and dates of birth of his children are all there

....
So it could well be that there are UK Military Records YET to be found that may well help your ongoing searching for Israel Brown. These current ones that you are eliminating should at least give you a good understanding of what to expect for Russians who enlisted in the UK military during WWI.
So when you do find your Israel Brown's Military Records you should expect to find confirming information about him, his wife and his children

... So don't give up the hunt for your Israel Brown.
Cheers, JM