On the idea of a dedicated trials ship for Sea Dart, how about one of the Tiger Class Cruisers? They were of dubious value as gun cruisers, with one of them laid up from 1962, why not take that ship and fit a prototype Sea Dart system to it, launcher and magazine in B position if possible, or otherwise replacing one of the Mk26 6" mounts. Being a cruiser there should have been sufficient space and weight for the rest of the system.
Who knows, if it worked perhaps the other two hulls could have been converted as well. Maybe Ikara could have been trialed on the same hull in some sort of midship arrangement replacing the beam 3" mounts. While I'm at it, if all three were converted in the late 60s, early 70s, hey could have also received Seacat and one of them could have been a trials ship for Sea Wolf. By 1982 all three could have had Sea Dart, Sea Wolf, Ikara and possibly even an aft flight deck and hangar for a pair of Seakings, while retaining the forward Mk26.