Madbob,
There are a number of reasons for not storing them; in no particular order:
1. To save money the military logistics system has moved from stockpiling vast amounts of spares "just in case" to a new system which is "just too late" by passing the onus onto industry to support our kit. The good thing for the beancounters is that it enabled them to dispose of large swathes of our depot system so there is not an awful lot of space left to store them, and spare HASs are dwindling as airfields close.
2. Related to point 1 is Resource Accounts Budgeting / Cost to Capital. In effect, every capital item the MoD owns (aircraft/tank/ship) costs the MoD 2.5% of its capital value per year to offset the money that the government has had to borrow to provide it. All assets do depreciate over time, but even a near lifex Jag will have a residual capital value. As will any hangar that you keep to put them in or kit you purchase to support it. RAB/C2C were designed to prevent Govt Depts "hoarding" assets, unfortunately this has removed much of the MoD's ability to surge/sustain campaigns at short notice.
3. Govts / Senior Officers don't like being proved wrong. Cut them up or flog them off and there's no chance of an embarressing future change of mind.
4. Protection of new toys. Do you honestly think anybody above Gp Capt in the RAF would risk the "my precious" Typhoon by keeping a couple of sqns of CAS ac at a time when the mud credentials of Typhoon are being pushed so much?
5. You're right. Unless I've missed it in 20 years of flying around the UK, there is not the climatic equivalent of AMARC anywhere. Maybe Cyprus, at a push??
6. The MoD are notoriously poor at realising good prices for surplus kit. Therefore, it's easier to dispose of them as a job lot and let a private company (such as Everett Aero) take the risk.
7. I may be wrong, but I recall that the F4s were scrapped due to CFE treaty restrictions. Maybe the Jag is also a treaty limited asset?