Hmm... when I get some time I might look into a Sea Slug conversion then and see what I can come up with.
I don't know how committed you are to Sea Slug, or all-British armament postwar, but personally, I think that Terrier might be a better way to go, and maybe even a bit more aesthetic, though I suppose that point might be debatable.
As for Seacat, I've honestly never exactly understood the point of a subsonic SAM, or why it lasted in service as long as it did. I mean, yes, I understand it's basically replacing the 40mm Bofors mounts, but by the time the thing entered service, it seems to have been obsolete. In your particular position, I'd just create your own AU alternate weapon system. Certainly a nation capable of producing such fine vessels can afford it!