Depends on the full circumstances, undertaking is bad especially if it's a bike sharing the lane with a car, also they shouldn't jump queues to the front (small queues I mean as it just makes the cars have to pass again, needless), long queues less of an issue why wait.
That said if I was planning to turn left in 20-30 metres and had to go past the cyclist first and was going to impede them I would have just stayed behind the cyclist. I certainly wouldn't have waited and let a car out, that's just causing it so you have some blame here.
Although I'll admit I rarely check my left mirror in single lane situations, if some cyclist want's to play to be silly with a 1 tonne car by undertaking and invoke Darwin's law then more fool them. But I wouldn't overtake a cyclist just before turning left, that would be me being silly and make it my fault.
Stuff shouldn't be going up your inside, so there is little need to be checking that mirror unless on a dual lane carriageway or motorway etc. Also as for blind spot checking, that's silly I'm more concerned with what is front of my than taking my eyes off the road to look behind / to the side, think it would be more dangerous to turn to check blind spots in a lot of situations. I just check my mirrors and consider speed well in advance of wanting to move so there is little need to turn to do a blind spot check (except for when reversing).