Wino
Thats an easy one, try the older laws!!!!!!
I keep saying this, and I will repeated again...Thats the only part of the ANO that I can find that refers to firearms. There may be something else, there may be agreements between the Goverment and the CAA. However I doubt it. Its 7 years since I worked at LHR as a Policeman, however at that stage the carrying of firearms by British Protection officers on British aircraft was not as you described it. I wont comment further on that.
Again to repeat, if this part of the ANO is what applies, and the Goverment decide to use skymarshals, then its a very simple matter to change, but at the moment it seems that its unlawful to carry armed skymarshals.