The new rank of Army Sergeant Major is held by Glenn Haughton, formerly WO1 in the Grenadier Guards
The Army creates its most senior post ever for a soldier - British Army Website
As you imply, the cap badge he is wearing is the badge worn by army staff officers and others who are ex regimentally employed. I have a photo of my father wearing this badge as a lieutenant in the Corps of Military Police in WW2, because at that time the CMP did not have any officers and they had to be seconded from other regiments and corps. If you read the link you will find that Sergeant Major Haughton is a member of the Executive Committee Army Board, as well as various other boards. An interesting fact is that at the end of his tenure, he will do the army staff course, and be commissioned in the rank of major.