I am sure that it will not take long to arrive at a consensus
non-political message. There would be no bar in personnel participating in this non-political march, just as there is no barrier to participating in other marches or - as acknowledged by ministers - from joining the British Armed Forces Federation.
I was on the Save The Scottish Regiments march and noticed quite a few serving types. I was also (briefly) on the Stop the War march in 2003 whilst serving, largely by coincidence as I had to meet my missus who was participating!
