I thought even uniforms would be worn on events like these..?! I mean, this was a purely military thing, wasn´t it?
In general, at these memorials of bygone military occasions veterans don't wear uniforms. (They were probably glad to get rid of them when they were demobbed in 1945!) At the WW2 commemorations last year in France I don't think Charles, Harry or William wore uniforms and most of the old soldiers didn't.