A lot of my remarks can also be applied to the UK in general, but my experiences tend to relate to England, and the interview that kicked it all off talked about 'Englishness' throughout.
I grew up in a typical English village, and it was very white, but not exclusively so. Even allowing for the fact that villages tend to have a fairly static population, there are plenty of reasons for people to move there or to visit, and not all of those people are going to be white. Off the top of my head new residents and short or longterm visitors in my experience had included health, legal and educational professionals, restaurant owners, friends and work colleagues of residents, new partners of residents (and any family they may bring with them to the relationship) and people who just want to live in the village. Not all of these people are going to look exactly like the people already there.
After the interview came out, I watched a few episodes of MM, having probably only seen one all the way through before, and the casting felt wrong. I think the whiteness may have been one of the reasons I hadn't got into the series before although I hadn't been able to adequately express that reason based on my small viewing sample.
As a contrast, Kate Ellis' Wesley Peterson novels (and I can't believe no one's considered dramatising them yet) are based around the idea of a black policeman and his family moving to a typical piece of rural England. They encounter a lot of white people, but they also meet people of different ethnicities, and Wesley's family have a habit of either dropping in or deciding to move to one of the other villages. The series has had a few problems with other diversity issues, but more on that later.
Currently I have a novel in revision, and a submitted short story set in rural England and with references to Wales in the latter. The novel quite deliberately had a main character of Black Caribbean descent, and the short story had a character who turned out to be mixed-race, although I hadn't originally planned her that way. Both stories touch on how characters respond to the idea of being different, but I spent far more time on what it means to move to a village from outside. In my experience that seems to be the bigger issue (although I've only observed from outside) especially when compared with the experiences people report of moving to a town or city as someone who is obviously different.
So obviously I disagree with the MM approach to 'Englishness' and can't believe that the Midsomers have never had so much as a non-white GP, dentist, teacher or police officer. Evidently also none of the residents have jobs in town or go to school in town where they have ethnic minority friends and colleagues that they can invite over to the village for a visit. Then again, considering what the creators want to do with other minorities, perhaps it's safer for the ethnic minority potential visitors that way...
The other part of that interview, which received less attention was the assertion that other minority groups (L and G were specifically mentioned, although I assume the B and the T would also be included) make for useful motives for murder along with 'incest and blackmail'. In other words, the creators want their LGBT characters to be victims or criminals rather than witnesses, investigating officers, friends and relatives of victims, or general bystanders. I'd almost prefer us not to be in there at all.
Note that nowhere in that has there been any mention of disability, although some forms of disability have been common in detective series since before I was born (detectives, and partners of detectives, as wheelchair users particularly). Have there been any characters with disabilities in MM or have they been whitewashed out as well?
Another irritating trend I've noticed in detective series recently, both on TV and in books is the trans* character as murderer. I can think of three without even trying and I'm sure there've been others. None of my three examples have had trans* characters in any other roles to balance things out and the implication in each has been that their criminal nature and trans* status are somehow bound together within a wider character flaw (incidentally all three have been transwomen: transmen don't seem to exist at all as far as the writers are concerned). Can anyone come up with a good counter-example here to cheer me?
So my closing questions are: do you notice when a cast is less diverse than the real world, or when characters only represent certain types of diversity if it's a plot point? Are you more forgiving if a series scores well on some forms of diversity but does badly on others? Do you have examples of where diversity in all forms is done well?