Southend: In the mass turbo-prop days SEN was very successful, it hasn't had a runway suitable for the B737/A320 jet age but, now, it has every chance of being a reasonable success.
Oxford: A flying club trying to make it as an airport but without a decent length of runway to do so.
Carlisle: This is an airport? The last airline scheduled service I recall to/from it was back in the 80's and Euroair's LHR-Carlisle-Dundee service ..... and that wasn't a roaring success!
Coventry: 15 minutes down a dual carriageway from BHX, what chance does it really have for an airline operation?
Bournemouth: Hurn has been around a lot longer than you have back beyond the days when BOAC operated their worldwide services from Hurn!
Norwich: How can this be marginal, it is the only operating airport in Norfolk and Suffolk, OK the A11 down to STN has improved over the years but who want to fly with a LCC out of STN!
Lydd: Besides 'puddle jumpers' across the channel this is not, I believe, an 'airline operations' airport.
Manston: Great for crew training, maintenance and some holiday charters or similar but too convenient to LGW to really make it and for many a year.
Gloucester: This is an 'airline operations' airport? Perhaps but only with a stretch of the imagination!
DTV/BLK/DSA: No comment.
Plymouth: A landing strip on top of a hill, next!