you would have two strong and very capable leaders which Britain could be proud of no matter who won.
Not with Johnson, I'm afraid.
Perhaps you have to live in this country to fully perceive this, but Johnson's public persona and political appeal is not how you describe it. He's as divisive as Donald Trump is or Stephen Harper was - utterly loathed by his ideological opponents and irritating to a wide swath of his own tribe as well. What you allege as his moderation and urbanity is based, as far as I can tell, on the fact that he was mayor of London. This might have been the case 5-10 years ago, but since the referendum he's pursued an avowedly hard-right/hard Brexit form of opportunism, making him toxic to centrists and cityfolk alike.
Also, being 'urban' isn't what it used to be. Again, 10 years ago when the conventional wisdom was that growth, diversity, cosmopolitanism, and unrestricted financial markets (read: London) were a good thing for everyone, Johnson's style of urbanism fit the bill. To centrist/moderate/urban/surburban voters he represented a bridgehead for the Tories: a sign that a stuffy, provincial, insular party could compete in the big, sexy, modern world of London (alongside the suave New Labourites who were in charge at the time). His pedigree made that case convincing: born abroad, Eton/Oxford educated, media savvy, and refreshingly unkempt when compared to the stolid figures elsewhere in the Tory party.
Now the mood has shifted. Super elite backgrounds and slick image management are much more cause for suspicion and derision, if not outright contempt, than previously. The reputation of London, its finance-dominated economy, and the wider system of inequality it helped perpetuate, are equally out of fashion. As as result, Johnson's profile doesn't come off as something people relate to anymore. His background looks ridiculously elite and out-of-touch. His 'buffoonery' comes off as a well practiced act (this is more due to him being around so long and the shtick wearing off). And his association with the policies of high neo-liberalism before the crash and austerity afterwards puts him at odds with the prevailing winds of populism, wealth redistribution, and renewed government intervention in the economy.
As for Corbyn and Khan, Corbyn is certainly divisive, but in the last election he pulled in as much of the electorate as Tony Blair did in his prime. He might not have much more room to grow with centrists or people over 45, but with the Tories imploding he doesn't really have to. Khan is a solid politician, but he lacks the charisma of Obama or Trudeau. To be honest, he has more in common with Sanders than Corbyn does - middle class background, hails from an ethnic minority, pragmatic rather than idealistic.
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