That is a suprise like, considering it's really a naval town...If they leave, until this Scottish defence force (see devils post above) what happens?
There's always been a (minority) undercurrent of ill-feeling toward the base as the MOD haven't exactly handled the local community that well at times, plus there's a lot of people who don't like living with their arses sat on the majority of the United Kingdom's nuclear weapons. The base used to bring a lot of benefits to local businesses but recently more and more contracts are not put out to local tender and have been going to large companies based elsewhere.
If it does happen I don't see a period of closure. It will take years to develop a deep-sea base somewhere on the west coast of England/Wales (or even Brittany has been mentioned). Most of the people there have civilian jobs with Babcocks (or other companies like that) with not that many naval positions - though there is a massive MOD police force on the sites.
The biggest problem for that area has always been NIMBYism more than anything else. So many projects that would have improved the place have been rejected cos of an attitude of snobbery really.