I think you can rest easy on this. Somewhere on this forum a long time ago
, Matt said I wouldn't be disappointed when I posted a 1st Dragoon helmet as the style of heavy unit helmet I would most like. Only the Dragoon Guards and Dragoons had that style with the flowing horse hair mane; the Life and Horse Guards had crests at that time. He has also said they would not do the Greys for a bit. We also know it will be a Waterloo unit which I think leaves the 1st Dragoon Guards, the 1st (Royals) Dragoons and the 6th (Inniskilling). Personally, I am rooting for the Royals since they were active in the Peninsula, as were the 3rd and 4th.
I wouldn't be surprised if it is the Dragoon Guards though since they were quite active at Waterloo against the Cuirassiers.
I can make that work too since the 3rd, 4th and 5th Dragoon Guards were all active in the Peninsula.