Undertaking / filtering on left?

That behaviour is standard on my commute. I'm one of the few who will drop back to let vehicles do their thing or slow down/stop to let vehicles emerge from side roads. Most of the time it seems like a waste as everybody else undertakes or overtakes.

Cycle training - so very useful, but I consider myself a commuting newbie of 18 months.

I've not had any cycle training* but it just seems retarded to undertake a vehicle indicating left.
Frustrating for those of us waiting behind the vehicle though as we have to wait much longer whilst someone wobbles slowly in and out of the way. Getting trapped behind a large vehicle that is moving slowly because it is stuck behind a selfish nodder is a pet hate.

*I have been driving for 8-9 years though.