Some Unis use 'accommodation' as a lever to get you to Firm them early - don't fall for this trick.
You are not going to be homeless, there is plenty of 1st year accommodation to go around and the vast majority of Universities do not use any sort of 'first come, first served' system. Most dont actually allocate rooms until after Results Day so the idea that you need to go into a flat-spin over this right now is all a bit silly.
The vast majority of applicants do not even have all 5 decisions back yet, so there really isnt the rush that some Unis will try to convince you exists. And if they really need to bribe applicants with 'special offers' to get them to go there, start asking yourself why they have problems recruiting students and why they need to do this.