Those airports on the coast that do prosper are likely to be close to a major conurbation, or serving an otherwise-isolated region: SEN (London/Essex), LEQ, NQY (isolated but hardly prospering), SOU (affluent Hampshire), NCL (Tyneside/NE). BRS (affluent westcountry), CWL (Cardiff/valleys), EXT (affluent Devon/isolated), GLA (Strathclyde).
However, I reiterate that it is noticeable that a significant proportion of the airports that have closed (in Britain) are coastal.