The decisions are made at the highest levels: AOC level.
There have been trends in recent years to preserve elderly Sqns. ie The lower the number the more likely to survive. Whilst one could argue that therefore No1 Sqn will be the final survivor to switch the lights off, there are influencing factors such as:
Sqns with history well known in the public domain: ie 617 Dambusters.
The fact that some of the most elderly sqns are ex coastal command and therefore 3 digit starting with '2' ie 216 / 230 rather then 1-2-3 etc.
Resurections of dormant Sqns are very rare although not unknown. 99 C17s comes to mind.
OCU/OCF were rebadged in a laudable attempt IMHO to retain Sqn identities.
Worth remembering that some wartime Sqn numbers were allocated to Czech, Dutch and Polish Sqns and I believe that some of these (now ex RAF) sqn numbers survive in their respective countries.
On occasions when the RAF is further reduced aircraft role and history in that role appears to influence number retention. Cynics will state that the background of certain senior officers having risen to the decision making levels are biased to their own Sqn history. Trends change over the years and it would be impossible to predict the number of the next sqn that will definately go.