In the north, it's Leeds (443,000) Bradford (293,000) and then Sunderland (280,000). But I'm not sure about Portsmouth, Southampton or Cardiff. Bradford used to have two league clubs (Park Avenue and City), of course, but doesn't any more. And back in the 1880s, Sunderland had two - Sunderland and Sunderland Albion. I don't think Leeds ever had two - Leeds City folded before the present club was formed.
That's what makes England football so different to Europe. Paris only has 1 big club, whereas London has 2 in the top . Manchester has 2, Marseille has 1 etc etc. Can you imagine how big a club London would have if you combined all of them together ?
Loads of Sunderland fans in South Tyneside, so I don't see how that would count just for the Mags... The biggest one club city in the UK is Leeds by a country mile...