"I wonder how much in some drivers' minds (subconsciously at least) there is the thought that 'cyclists only need a small amount of space to get through traffic, so I only need to leave them a small amount to pass safely'"
I've often wondered the same thing. I'll filter through gaps that are much much smaller than I'm happy having a car pass me by. I think there's a difference in that the differential speed is much lower, and the capacity for harm so much lower (I could scratch paintwork; they could kill me), but I cna see how me going throug a small gap would equate in many minds to that being the amount of space a cyclist needs.
Of course I wish the Highway Code would be more explicit on this and actually just say leave a 3 feet gap or something like that.