If more pistols were required, they really aren't all that expensive. The 25000 Glock 17s were GBP 9M. Say another 75000 were required, it would be about 25M, given the size of the order. Culturally however, it is unlikely to happen in the present climate.
Perhaps another way to approach this, is to allow servicemen to apply for the right to carry, through a very thorough training course and marksmanship exam. Make it impossible to pass without understanding the ramifications of shooting someone innocent under personal protection circumstances, and make sure they are proficient with the weapon. That way, those that feel genuinely threatened can be offered a route to a degree of comfort, and thus confidence. Lots of issues to be addressed, but terrorist abduction of servicemen changes things.