Whilst Spain's fleet has improved it still doesn't have force multipliers comparable to the RN, namely SSN's and SSBN's, which you have already mentioned. The Astute platform is a step change over its predecessors and brings to the table a level of threat (stealth, endurance and weapons load) against a potential foe most Western Navies could only dream of. Such expensive investments will ultimately have an impact on the amount of money available for surface ships, however a single modern SSN sub can confine an entire surface fleet to port, so they bring a huge amount of clout to the table.
We have got to stop counting hulls and look at how our assets can impact the overall outcome of a battle. If we continue to face asymmetrical threats then we really need to maintain the ability to strike against our enemy on land and sea. Talking about withdrawing Sea Harrier in the current environment is irrelevant, GR9's bring to the table what we need in the form of a very capable CAS platform, the old Sea Harrier would contribute zero to the current battlefield other than look good sitting on the deck of an Invincible class.
With the arrival of the six T45's we have pretty damn good coverage to deal with rogue states attempting to fire the odd silkworm at the amphib fleet in the littoral environment, which is a far more likely scenario than witnesseing another 1982 surge down South to take back our static aircraft carrier, which now have the following anyway:
Typhoons Depart for the Falklands
There are no states out there currently looking to kick-off a full-on convential war aginst the UK, so quite rightly the UK will continue to focus on the here and now, which last time I looked was winning the assymetrical war in Afghanistan supported by CAS not CAP.
The following link provides an update on the RN's current priorities.
http://www.royalnavy.mod.uk/upload/pdf/RNGF09_sep09_72dpi.pdf