Blanket bans are counter-productive. Try to keep everything secret, and you just stimulate curiosity and encourage the bloody-minded to dig deeper.
Moreover, it ought to be a guiding principal that (except where it is demonstrably and materially damaging to do otherwise) the taxpayer does have some right to know how his/her money is being spent, and indeed what is being done in his or her name. Go too far from this and it's counter productive as above.
Nothing should EVER be protected on security grounds when all that is being protected is political embarrassment or the reputation of a senior officer who has acted unwisely or improperly.
However, that does not mean that there should be carte blanche for journos, nor even that anything should be fair game unless it can be proven to be damaging.
There is a sensible, commonsense, middle ground, and there are some general rules which you'd find most journos willing to stick to.
No parametrics (especially not weapons and sensor performance).
No tactics. (Doctrine is fair game, though).
No names of people serving in operational theatres.
No names of current overseas op locations except where previously released and confirmed by MoD.
No arrival/departure dates until after the event, and after released and confirmed by MoD.
Anything else should be on a case-by-case basis, guided by commonsense, and with caution and conservatism the guiding principals.