Ignoring the press hyberbole, if it's anywhere outside our 'territorial' waters but still considerably closer than any ICBM launch site, the knock-out punch can still be delivered, not having any MPA just means that our ability to prosecute the aggressor is considerably diminished.
Highly unlikely scenario in any case, assuming it is a russian sub, it's just some more willy waving from uncle putin.
RP, we respond to the bears because we can - simples. Also good practice for other scenario's, it's unlikely that IS will hijack a nuclear sub.