According to this site it depends:
www.jdawiseman.com/papers/union-jack/union-jack.html.
I know how you feel. A neighbour told me a few years ago. I've only ever heard it refered to as the Union Jack. Everyone calls it the Union Jack anyway.
Going slightly of topic, I can't really remember our local church flying it from the mast, it's usually St George's Cross.
Anyway, yes, apparently it's only the Union Jack when it's on a ship.