Its all done by Natural England. They have a strategy for reviewing what habitats and species are of importance. which changes over time. They work with other nature bodies and organisations to prioritise what needs designating. It might be one year a certain bird species goes on the red list, so they look to identify and protect its nest sites, other species may recover and be de-designated. Its complex and bureaucratic.
Its also rather meaningless if we have a government which totally ignores SSSI designation, reverses the policy it declared before the election and includes SSSI's and AONB's in areas to which it grants fracking licences. They seem to think money and industry comes first and environment comes a distant second. Its disgraceful, our ecology is very much at risk by this blinkered shower. Mr Angry of Somerset, quite disgusted by their ignoring their own statutory guidelines to protect our natural heritage.
http://www.theguardian.com/environment/2015/jul/16/government-makes-outrageous-u-turn-over-fracking-in-precious-wildlife-sites