Manchester and Birmingham accommodation applications already open but not necessarily first come first served.
At least last year (and when DS1 was applying for 2015 entry), at Birmingjam, you submit your preferences, then halls are allocated from sometime in July onwards for Uncondotional offer holders and after results day, I think, for those with conditional offers. Both of ours were in the former category- DD had to inform the university that I had her permission to act on her behalf, as she was going off to Madagascar just before she was due to get her allocation. You have to accept and pay the deposit within a few days.