waremark wrote:I should have given more thought to the lack of options in view of the car downhill towards me, and I should have been more focused on considering up to what point I could safely abort the overtake if the van moved out.
Those both sound like sensible thoughts.
waremark wrote:I don't think it would have been appropriate to use lights or horn in advance
But I wonder why you say this? Rather than relying on the other driver to remember to check their mirror, could using the lights or horn in advance be taking a proactive step to try and make sure they know you are there?
waremark wrote:(the car has daylight running lights - I think it is obvious the van driver did not look in his door mirror before starting to pull out).
Perhaps a headlamp flash as you approached - the light coming on and going off again - might have caught the van driver's eye and drawn their attention to your presence in a way that the constant light from your DRLs is maybe less likely to.
It's got nothing to do with the speed limit aspect of your post, but...
I had an incident fairly recently where a vehicle I was planning to overtake pulled out in front of me. My situation wasn't exactly the same as yours, but like you I was to their offside and behind, with probably a similar sort of closing speed, and like you I had the impression that the other driver didn't check her mirrors. It wasn't quite as dramatic as yours sounds - I was able to brake and stay behind. Since then I've thought that when planning to overtake and approaching with a significant speed differential, a precautionary headlamp flash prior to the point of no return is worth seriously considering.