Each line has it's own rules for promotional rates. I am not aware of any line that offers military promotions other than for North American passengers. Some are very restrictive even for US and Canadian military. HAL for instance, is available to active duty members only. Some are for active duty, 20 year retired or two years duty during war time. Carnival is very inclusive with their rates allowing anyone active, retired or even non-retired veterans with honorable discharge papers called Form DD-214.
Perhaps members of the British armed forces and other European services should ask the lines to include them. You may have better odds of getting P&O, MSC or other European cruiselines catering heavily to European passengers to consider such a policy. The shorter answer would have been, I simply don't know why the rules are set up the way they are.