As Earl of Wessex Edward takes the honorary title of Colonel in Chief of The Wessex Yeomanry. He is entitled to parade in that uniform. (of all the Royal he's the one I dislike, as seems apt today, he's the Blackadder of the first series, toadying, obsequious turd!)
The Duke of York is Colonel-in-Chief of the 9th/12th Royal Lancers, the Royal Irish Regiment (27th (Inniskilling) 83rd and 87th), the Small Arms School Corps, the Yorkshire Regiment (14th/15th, 19th and 33rd/76th Foot) and Colonel of the Royal Highland Fusiliers, 2nd Btn Royal Regiment of Scotland.
By the same rights as his brother he can wear the uniforms of any of the titles above and has done in the past, I assume respecting his Naval career he respect the service division and does not parade in uniform as Trooping the Colour is essentially an Army tradition and an Army Parade.
In my view he has the right to wear any Uniform with pride as a retired, active line officer, like Charles and the two currently serving Princes, Edward couldn't even pass out at Sandhurst!
Just my views