my late grandfather was discharged from army on 28-6-1921
Though he was still in the army, William Jones should be in the 1921 census. Gunner in the RGA, I guess.
You would think he might be in England (or the UK, at any rate), about to be discharged?

There's a William Jones in Richmond, Yorks, born 1884, Manchester (RGA?)
There are others, including abroad, i.e. one born 1887, Birmingham, in the Royal Artillery?
For those of us without full access, it's very hard to look.