Contracting out leads to contraction of your pool of deployable personnel. Sqns aren't too bad, but what about OOA VAHS sections or all the other supporting roles that make an airfield tick, these tend to be made up of waifs and strays from all those second line bays etc. Bin them and who's going to do the job, the hard pressed sqn guys? And is it really sensible to pull them away from their specialisation to do a generalist task? These days to meet the mandated requirement for Suitably Qualified and Experienced Personnel takes considerable investment at each rank level. That is a tension that the Service cannot resist.
There is a manning problem, TG1 are about to go to 12m PVR notice; not the sign of a healthy manning balance. Some other TGs are in much worse condition. FF2020 manning requirements have already been breached. The 'offer' is no longer enough for a lot of good guys; I've had people who are going to work as train drivers, or renewable techs, or train maintainers, or going into teaching, or maintaining production line equipment. They are all looking at 50-100% salary increase + overtime and they all remember the trauma of the redundancy notices less than 5 years ago.