Regular Service personnel are not to take an active part in the affairs of any political organisation, party or movement. They are not to participate in political marches
...but it's ok as long as it's a 'Gay Rights' parade, in uniform...
Precedent set, methinks.
How about this for sending a message: March, en masse, up the Mall, face the palace, have a band play the National Anthem during a General Salute; them march down to No 10, form up, face AWAY from the door and.....2 minute silence. That'll send a strong message...