Ragna,
My grandfather served in the Second World War as a young married man so he was closer to our times. I had to provide a copy of his death certificate to prove he had been dead 20 years (or whatever the requirement is here in Canada; I can't remember). But now I'm afraid to order my other grandfather's records from the Second World War, LOL!
If Sydney was enumerated by his name (and it was spelled and then transcribed correctly), then it should come up, no matter where he was.
One day you might end up going through the Battersea census page by page the way I've done for some American and Canadian cities and towns. If you do, record your notes in a notebook, so you never lose them.
If he was born about 1867, he should have been enumerated somewhere in 1871, 1881, 1891 & 1901. The fact that we haven't been able to find him in even one of these makes me wonder if, as you said, he changed his name, or his name has been totally mistranscribed each time, or he was in institutions his whole life and was always listed by his initials. But I tried searching on S G F and none of the surnames seemed to fit. But that's assuming they would have used all three initials. Even if he moved to England the day before his wedding, he should be somewhere nearby in 1901, since he and his wife continued to have children together. And he was using the surname McLaven by then. I wouldn't expect a hotel clerk to have to travel for his work.
I'm in Toronto. Maybe Sydney George Frederick has decided to haunt me because the Ontario McLavens aren't responding to his nudges.

Regards,
Josephine