From there I'd see where his parents were in the 1911 census. If it's the same location, odds are high that's where he was born.

Exceptions are possible. To help rule them out, I'd research and build-out the extended family. In that day, siblings/cousins banded together; local migration shows up when viewing the lateral family.

Is it not possible that there really was a John Alfred Tinniswood living in 1921 but the man who now claims to be him in 2024 is actually someone else who stole the original's identity when he died unrecorded?

Liverpool was the second city of the British empire at this point, so again, its “roughness” could be debatable.