I tried to remember to look out for this at my polling station. Since the church hall was completely remodelled a couple of years ago, it now looks like it's genuinely step-free access. The doors are fully open, and are wide enough to get a power chair in. There are lower-height booths, (though the clearance underneath is limited, so it's probably bloody awkward to get a scooter or bigger chair close in), but I'm not completely sure it would have been easy to reverse out without running something over. (Mind, actually getting to the polling station is another matter - the pavements nearby are very narrow, and it was bin day, so obstacles all over the place. (You too can complete the slalom before voting!)
Not sure how easy it would be for people with other disabilities.