It's filtering, not undertaking, and is not only perfectly legal it's often exactly where the cycle lane is placed. It's up to drivers to look before turning, but cyclists injudiciously filtering is not how they are getting hurt, it's either rear impacts of a left hook, the cyclist did nothing whatsoever wrong.
You said it pretty well, the problem is not people filtering on the inside, but for decades we (from society, to government) constantly insist that they should always hug the kerbs.
Then when some people got killed as a result, we blame them for doing so.
It's filtering, not undertaking, and is not only perfectly legal it's often exactly where the cycle lane is placed. It's up to drivers to look before turning, but cyclists injudiciously filtering is not how they are getting hurt, it's either rear impacts of a left hook, the cyclist did nothing whatsoever wrong.