You need to read on each uni accommodation page. Approaches differ and vary year-to-year.
Some allow all offer holders to rank accommodation choices (Durham asks all offer holders to rank colleges) and others allow those who have firmed or insured. I know York does this. Other places only let Firm candidates rank their choices.
Note some places, especially privately owned halls rather than uni owned might allocate specific accommodation as applications come in. Private halls often ask for immediate deposit. There seem to be more of these at post 92 unis.
Others will give an allocation before results and many only allocate once results are in.
I know Warwick asks students to express preferences by about 3 days after results which means firms, those who come as insurance or clearing all have same opportunities for accom.
Quite a few places won’t guarantee accom for the nsurance applicants but will for those in Clearing as a marketing technique to fill spaces.
It is difficult to have certainty about specific accommodation choices. It’s best for students to remain flexible in their thinking and not get too attached to any one accom.
Definitely mak sure deadlines are met and email in spam checked. One way unis manage accom over subscription is by having clauses that state those who miss deadlines aren’t guaranteed. They will try to give them uni accom but if there isn’t enough, those who missed deadlines miss out. Some people need to chase their rather laid back terms to ensure this doesn’t happen.
Check for later deadlines too. Durham has one date for ranking college preferences but another for requesting accommodation.
It’s all doable but requires a bit of attention to it.