Thanks, that's very informative. That 8.8-inch envelope diameter rules out a simple adaptation of a W54/Scarab style primary, which was about 10.8 inches in diameter and likely could not be shrunk much further. You'd probably be looking at a warhead derived from artillery shell design instead. And those don't seem to be light enough, though a portion of that may be due to shock hardening for artillery launch.
IMHO, of course, but any kind of nuclear SAM that Royal Navy might field during Cold War would likely be a nuclear derivative of Seaslug:
* The Seaslug became operational exactly in time period, when nuclear-tipped SAM's were on the peak of popularity, and thus the idea "Royal Navy must also have one" would be absolutely plausible. By the time Sea Dart become operational in early 1970s, the nuclear-tipped SAM's weren't as popular anymore (both due to political concerns and the simple fact that radar & guidance technology improved enough for concentional SAM's to do the job);
* The Seaslug is a large missile with large warhead section. It could easily be adapted to fit at least several variations of nuclear warheads. The most likely version would probably be a lightened WE.117A design; it seems to be a right diameter to fit inside missile body (albeit tightly), and it was already used by RN as nuclear depth charge;
* The Seaslug is a beam-rider missile. It's essentially immune to jamming, chaffs, decoys; as long as Type 901 radar beam is on target, Seaslug would hit it. For a tactical nuclear weapon, the immunity to enemy countermeasures was always highly desirable (there are always the chance that decoyed missile might fly too close to friendly unit...). Note that nuclear version of both RIM-2 Terrier and RIM-8 Talos were pure beam-riders, without homing guidance;
So basically, if RN really wanted a nuclear SAM, and managed to persuade government to fund it in 1960s - it would likely be a nuclear-tipped Seaslug Mk.3, with WE.117A warhead.
P.S. One interesting note is, that Seaslug actually didn't need nuclear warhead much. The main reason for the nuclear-tipped Terrier and Talos missiles was, that 1950s radars could not discriminate between several targets in close formation; it was possible that they would start to track the whole formation as one big target, and missiles would just fly between enemy planes. The nuclear warheads were supposed to solve this problem, by either destroying all formation at once, or forcing enemy planes to disperce (so they could be picked up by conventional missiles).
Well, the Seaslug guidance radar did not have such limitation. The Type 901 guidance beam was so narrow, that the radar required a specific search pattern to just lock on the target (the data from usual ship tracking & heigh-finder radars was too inaccurate for high-precision beam!) Its azimuth resolution was about 0.33 degrees, and range resolution was less than 70 meters. So Seaslug could just discriminate between target in close formation by itself; no need to go nuclear.
