Well when I was researching populations I was surprises to see that MK is now well over 200,000. Yet I believe it's still not a city. However it sure has grown!
Technically Sheffield has a larger population than Manchester, although it depends what criteria you use. I don't know if that includes Salford, Stretford, etc. Obviously Greater Manchester is much larger.
The population league tables is quite complex. One version has Wakefield, which I believe is only a town and doesn't even a league football team, in the top dozen with a population of over 300,000.
When I was a kid in the sixties I used to read my dad's RAC book and the relative size of English cities was much more straightforward back then.
1. London.
2. Birmingham.
3. Manchester.
4. Liverpool.
5. Leeds.
6. Sheffield.
It is much more complex now and Sheffield certainly now has a larger population than Liverpool which has shrunk. It is also possibly now slightly larger than Leeds. The only certainty is that Leeds is easily the largest city with just one football team!