Another issue is that sufficient spares just don't exist, in almost any flying military organization.
Supply priority generally goes to deployed aircraft. Because of that many (or most) of the few available parts are in the pipeline either heading out as installable or being retrograded as repairables. Pipeline time is dead time as nothing can be done with them until they arrive on either end. The solution is obviously to have bought more spares to begin with but convince the peacetime-minded beancounters of that.
It's not unusual to have to canibalize the home fleet extensively to feed that pipeline and that's where availability numbers really go to !!!!e.
That certainly isn't a RAF specific issue. The US Army and USMC are having similar problems. Lots of aircraft sitting on the ramp at home missing parts and lots of material being shipped back and forth.