Highway Code rule 170, pedestrians already crossing a side road, into which you are turning have the right of way and drivers must wait for them to cross. But clearly pedestrians shouldn't simply step out without checking it's safe to do so.
That's always been my understanding.
For a brief few years in the late 70's motorcycles were required to be fitted with an audible beeper when the indicators were in use for the obvious reason that it would be a safety reminder to turn them off. I find the beeper on my 79 BMW useful for another purpose. If I'm approaching a left turn any pedestrians who are still some yards from beginning to cross a side road are warned of my approach and intention. When I see the heads turn in response to the beeper I know they are aware of my approach and will generally not walk out into the road while I'm making the turn. If they have already begun to cross the road then of course I'll slow or stop.