You have to remember that COTS rarely applies to any RN ship.
A mildly odd example was HMY Britannia. She was fueled by heavy bunker fuel, not diesel, so could not be deployed in wartime without her own dedicated tanker, and her AC supply was generated by diesel engines that had been standard fit for wartime submarines.
Just re-plumbing a warship for an up-to-date C3I fit is a nightmare.