To turn to the FAC issue - which I still don't believe is viable, for the RN it is I suppose outside the box thinking, but I cannot for the life of me think which threats we face, or will conceivably face where Visby's or whatever become the best option.
Anti-Piracy/Maritime counter terrorism - no, helo's and probably Bay, more likely DDG/FFG perform this role better
MCM - no, er thats more suited to MCMV funnily enough
Coastal patrolling - we don't realistically face a maritime threat to the mainland UK in the next decade, and don't have the necessary support structures in place for FAC's to operate to their full capability. The Rivers are good enough for now although more would be nicer (as ever)
Possible SLOC protection from an emerging Iranian threat, bolstered by their increased numbers of cheap land based SSM, a second 'tanker war' perhaps - as is their 'interesting' notion of swarm attacks by FIAC's (
http://defensetech.org/2010/04/26/hybrid-war-at-sea-irans-great-prophet-5-exercises/#more-6744) in line with "tekirdag" - Now here I see a viable use and the possibility of real use being had, yet FFG/DDG could deal with as well, and be more versatile elsewhere
Confrontation with North Korea - ignoring the difficulties of transporting our squadron of FACs halfway around the world, what possible offensive use could we have for them, what targets that won't already have been bombed into oblivion by massed US carrier aircraft, ROK attacks, harpoon strikes, SSN attacks, sea skua/penguin armed helo's, even 4.5". By the time our little FAC's are close enough... I don't see much of a force left to strike.
Falklands... well, how much use is a FAC down south? and how many would we lose just through damage from sea state?
note that in this i'm using FAC in the conventional terms, not Tekirdags ridiculous aircraft-of-the-sea approach.