FV432/30 was top heavy, had a cramped interior and might have meant no Warrior, so cancellation after 13 examples was a good decision, IMVHO. LWT Sea Wolf was a sad loss.
The armament of the Castle Class OPV would have been the ROF 105mm, as used on the Centurion, but not the turret. It was the cheapest option until the MoD 'improved' it so that it became the most expensive option and the class got the 40mm Bofors instead. It would not have been used on the Island class at those ships predated the idea.
DK Brown discusses the whole project in Rebuilding the Royal Navy.