It's not a missed opportunity, it's the entire point of the show! The whole 'fish out of water' vibe and how these assorted, but very British, types adjust or fail to adjust to these very different corcumstances. If you wanted to have a Black lead, then you'd have to have either a very Black British, Trevor MacDonald, Clive Myrie type - at which point everyone would be up in arms about the character not being authentically Black, or some street-wise, hard-bitten Brixton character, at which point the show becomes edgy and no longer a light-hearted whodunnit.