This was an interesting instance:
I will give a similar anecdotal instance that happened to me and almost identical to that (all speeds are assumed given my Cruise Control setting as you will see in description):
3 Lane motorway, speed limit is 50 for this stretch. There is a car (Car 1) in lane 2 doing circa 45-46MPH and the police are behind them, also in lane 2.
I am in lane 1, slowly catching up with Car 1 and Cop Car in lane 2, and stick the cruise control on at 50mph. I did this so I didnt get pulled as the cop car was a traffic car. The next bit happens over 2-3 mins of driving (so about 2 miles?)
As I am going a few mph more than Car 1 (and therefore Cop Car behind Car 1), I slowly start to pass the cop car on their left (I am lane 1 they are lane 2). I pass cop car with no dramas.
As I draw almost level with car 1 (bonnet in line with their NSF door, the cop car puts on their lights behind car 1 who proceeds to panic and swerve a bit into lane 1 (where I am). Thankfully there is a hard shoulder which I can move into and I am being vigilant for this kind of manoeuvre
Cop car continues to follow Car 1 with blue lights on, pulling them over once I have cleared them (still in lane 1)
Presumably they were getting pulled over for lane hogging in Lane 2 but probably escalated to a sterner waggy finger by not looking before switching lanes and almost causing a collision.
Anyways, my point is this - this cop had no issue with me "undertaking" but did have issue with Car 1 middle lane hogging (cop car could have passed in Lane 3 if it wanted but stayed in lane 2 which, I assume, was to try and "encourage" car 1 to move over properly.)
Except in slow moving traffic/roadworks - it is practically impossible to be undertaken if you are following correct lane discipline! So anyone that gets upset about it should have a look at their part in allowing the uindertake.