I agree that with the current & forecast size of the fleet 3 Naval bases is too many. Trouble is that the infrastructure has grown with a haphazard regard for efficient planning.
Faslane has the deterrent & Coulport so needs to stay open.
Devonport has the N Sub refit complex so probably needs to stay open
Portsmouth has many of the shore training establishments and the Naval HQ
And, as an aside, Rosyth needs to be kept open as its the only dockyard that can refit the (new) carriers.
Should probably have closed Portsmouth, moved the training establishments to Guzz & done what they did to Vernon site - developed it as the RN museum & a plush waterfront housing site. Allowing VT (now BAe shipbuilders or whatever) to relocate their ship building facilities there muddied those waters. Trouble is, that this would all cost money in the short term & that we don't have.