There are some things that can constitute careless driving, and some that invariably will constitute careless driving. Those that can tend to be careless when done without adequate care. Those that will tend to be considered careless in any circumstances.
As part of the IAM advanced motorcycle test, riders are expected to filter through traffic carefully when appropriate. Filtering is both undertaking and overtaking at the same time. The whole IAM thing is based on Roadcraft - the police advanced driving manual (albeit translated into English) - so if anything, the expected disciplines are higher than the standard expected of the average careful and competent rider.
Obviously, that does not mean that a driver undertaking a police car in a campervan is a better driver than the driver of the police car, or that undertaking would never constitute careless driving, but it does appear to call into question any assertion that undertaking invariably constitutes careless driving.
edit: As some may be aware, I am not a fan of undue brevity, and therefore perhaps not best placed to read between the single line of those who for some reason are, but if I were to briefly describe an incident involving a f*ck off great 3 tonne campervan bearing down on me at great speed, swerving into the inside lane, undertaking me, and then cutting me up swerving back into lane 2 in front of me, I think that I would use a more descriptive term than "vehicle bearing the VRM #########" to describe said campervan. That said, if I had contributed to the incident by cutting up the campervan in the first instance, then I might be inclined to omit as much detail as possible.
