You need a lot of Type 45s to give the same coverage as a naval air-defence fighter. - Admiral Sir Alan West, in
this interview back in 2003.
I put a link to this one on the first page of the
Sea Jet thread (a long, long time ago
).
I suspect the reason that they are fitting Phalanx instead of Goalkeeper is due to cost, and being able to use the systems taken from the Type 42s as they decommision.