In truth, a lot of things will be learned by having these elections on different days. I'd be really surprised if the PCC elections get more than about 15% across the country. Areas where there are also local elections will get a little more, but that will depend on local stuff - in a borough near me, there's been a load of trouble over the past year with traffic issues, which the population blame on pathetically inept decisions by the incumbent councillors. I'd expect that area to have a higher than average turnout.
London Mayoral elections will always generate voters, although it's gonna be Labour or Tory, so the outcome in this AV election will be decided by the candidate the other party voters dislike the least. Not necessarily the way politicians should be given such power.
This does give them a pretty wide range of data (PCC only, PCC plus local, PCC plus Mayors, etc). The EU referendum is bound to generate votes, and a high turnout is definitely on the cards. If they held it on the same day as the others, those results may well get skewed (ironically) by a large number of people sticking a cross against a name without caring at all who they are voting for.