This does beg the question "by whom?". I struggle to think of any nation which has a beef with the UK and a sufficiently capable naval fleet to be able to mount a seaborne attack. As for a terrorist attack, well, we could have a fleet the size of the yanks and you could still wouldn't be able to protect all of our coast all of the time. Sounds to me this is the kind of 'issue' posed by a group of MPs with a bit too much time on their hands.
Also, why hasn't anyone mentioned the 4th emergency service (no, not the AA) the Coastguard? They've got air & sea assets which could be armed in extremis. I'm sure you could dangle an Exocet or two off their shiny new AW139s.