We were never really supposed to reserve phones full stop, but at my store we used to keep them for a few hours if people were popping down - no longer than that though.
Stores have been told not to reserve iPhones for people - although your mileage is clearly going to differ with managers at different stores maybe diverting from the rules slightly.
From what I've heard about stock allocation though you'll be fine with going to an o2 store on the day.