You really do seem obsessed with race and culture!
Oxford is culturally diverse, junior staff will reflect this and senior staff are likely to transfer in from other areas as per the new head honcho who has come from London.
when you say increase.....how many more are in the current series than in previous series? And are they in major roles, or just extras? I never watch it!
If I've followed it properly, Lewis resigned but changed his mind and came back. Not sure if it's a new series, but I haven't seen any of those currently on.
There are less and less coppers on the beat - and going to be even fewer - and yet high ranking Lewis and his chum seem to be always walking about hither and thither, I wonder where they are bound, perhaps they easily get lost.
/// What if they all got parts as the bad guys, would he campaign against that as well? ///
Most likely, but I doubt you will see it, because in all the crime series that contain an increasing number of black actors, it is very rare for them to appear in a criminal role.
Even in the real life documentary series about UK police work, I have yet to see them apprehend or arrest anyone apart from whites.