I cannot see that the British government would use anyone but police officers for 'sky marshal' duties ( an unfortunate term as it does not relate to anything in the official British psyche).
If this is the case then I believe that the officers would be from the Metropolitan Police SO 19 branch. This unit has some of the most highly trained and proficient armed police officers in the world.
So far I am reasonably content in that if there must be armed personnel on British aircraft then there could not be any better for this purpose than those from SO 19.
However, I am not a pilot, never have been, neither have I worked in the aviation industry. I must therefore bow to the superior knowledge of those who do/have done so, and it appears there is a real danger that an armed officer (no matter how proficient and well-trained) could be more of a danger than terrorists themselves.
Some posters have said that we should ensure that no terrorist ever gets on board an aircraft where he/she can ply his/her trade. No matter how many systems are put in place, the bottom line is that there is a human input to security at the very sharpest end, and humans are fallible, as are of course 'sky marshals'.
So, is today's announcement a sop (but to who?) and/or mere window dressing?
I don't know. What I do know though is that the more posts I read in this thread the more confused I become.