I could be adding 2 and 2 and getting 5 anyhow here goes. Several years ago the French tested a warhead in French Polynesia atmospherically in breach of the rules. Everybody was in uproar about this but the UK was surprisingly quiet. Now is it possible that the bucket of sunshine was a joint UK/French job ? The reason I say this is both the French and UK have potentially a similar platform to put this on.
The French have SCALP we have Storm Shadow which I think are the same unit made by MBDA
So if the French warhead tested was for SCALP then it would fit Storm Shadow.
Now allowing for the storage and control issues etc I believe you have a suitable weapon for the RAF.