Nab
Not trying to be picky, but I am guessing that S3 wasn't considered because it went of service in 2009.
And of course this HCDC report isn't just about ASW, it's Maritime Surveillance. Carrier based MS assets might be OK close in to the carrier, but they aren't much good if you are trying to patrol the UK's waters or provide ASuW coverage off Somaila but with no carriers around/available.