Insty,
12 will only go if it is decided to lose a GR4 unit from Lossie. If it's decided to shuffle units about (i.e. move one squadron from Lossie to Marham) and lose one, then one of the junior plates will disappear instead (31). Although it's not meant to, role might come into it - but as 13 is senior to 31, it probably wouldn't affect the outcome.
I believe that there was a cunning plan for the disbandment and reformation of Jag/F3/Typhoon units (plan without details mentioned to the Commons Defence Cttee), but this could all go out of the window if the number of Typhoon sqs is cut. Plate allocations can change right up to the point of announcement. The F-4J(UK) sqn plate was meant to be 39 until it was decided that a more feline plate should be adopted....
41 may well survive, as it is senior to 111 and 43. 54's chances look less happy.
If it's any help, the seniority (roughly) for the extant Jag & F3 plates is:
6
11
25
41
56
16
111
54
43
Before anyone wonders, this is public domain stuff - for anyone sad enough to look in the PRO!
Although those in charge of the numberplating probably have a very good idea of which plates will be used, keeping the punters guessing seems a favourite trick. And as impiger notes, if the CAS of the time happens to have been with 43 and not, say, 56, when the time comes to form a unit....