I too gave up reading Martha Grimes a few decades ago, and one of the reasons was that, as much as she evidently prided herself on picking up BE expressions, there were just too many cases like that where her mishmash of dialogue was distracting, even to me as an AE speaker.
Unfortunately, I get the impression that fewer and fewer Americans now read British novels, watch British TV shows on PBS, etc. In recent years, the American closed captions (= optional subtitles) on BBC, ITV, etc.-imported TV shows, whether dramas, mysteries, comedies, nature programs, etc., are frequently hilariously bad. I seem to recall that they're often done at the Boston PBS station, but apparently they hardly ever hire anyone actually familiar with British English, which you would think they could find if they tried, in a city with so many universities. Or these days, just pay someone in Britain, and get the spelling right at the same time.
The most glaring example is that 'mum' for a female parent is inevitably transcribed 'mom.'
One last night -- I think it was on Father Brown, but it might have been young Morse or even that awful soap opera 'Jamestown' -- was 'bobble' written for 'bauble.'
I wish I could remember some others.