It is a Union Flag or a "Jack"
The 'PC' brigade have been trying to get us Brits to call it a Union Flag when on Land and Union Jack is OK when on the Sea.
Being very very 'Old School' myself, It was always a 'Union Jack' in my Father's Navy Day's so that has always been good enough for me .
The rest can go and hoist their own whatsits.
After all, It was also the Submariners who rebelled and introduced use of the Jolly Roger or Skull & Crossbones as well when some old officer type said they all looked like a bunch of Pirates in their dirty white Sweaters and Unshaven appearance
