The fact that no-one has accidentally disacharged a firearm on a UK aircraft to date is almost certainly due to the small number of people who have actually been involved. Close Protection personnel unload there firearms before boarding (in the VERY rare circumstances that they actually carry them abroad). This would not be the case with sky marshals. As someone who spent 20 years of his life carrying firearms on a daily basis, and training others to use them, I know that the reality is that the longer the time since you let a round off accidentally, the nearer it is to the next one! I know of a number of highly trained police firearms officers in the UK who have accidentally shot themselves, inanimate objects and each other due to brain-fade. (At least 2 of these incidents were at an airport.) Most ex-military specialists now working as professional bodyguards will also admit privately to having at least one 'ND' in their careers too. Sky Marshals in the US have an excellent record though, but have had the advantage of developing in an environment where handgun training has been carried out from an early stage in their careers. The policemen that Darling is proposing to do this job haven't!
There is no magic cure for the problems of terrorists on aircraft. By far the best option is to stop them getting aboard in the first place, but this is never going to be the waterproof option. Having actually trained in mock-up aircraft I know how difficult it is to avoid a 'shoot-through' and there would probably be 'good-guy' casualties (although not as many as in a suicide attack designed to bring an aircraft down). The problem about frangible ammunition, such as the Glaser Safety Slug, is that it has weaknesses and is not the 'magic bullet' that the media suggests. I suspect that if a serious risk assessment was acrried out, it would result in the discovery that there would be more civilian casualties from accidents from armed marshals than from preventable on-board terrorist activities. In reality I think this announcement is nothing more then 'spin' to take the heat off the government at a difficult time.