The article that spawned the question : Boom Times
Boom rigs were relatively hard to find in New Zealand, although they are now more common in tackle shops close to a deep water fishery.
I hesitate to recommend any shops because sometimes tackle stores them have them - sometimes they don't. Best bet, let your fingers do the walking, on the phone or on the keyboard on the web.
You are almost certain to find supplies in Tackle shops dealing in Saltwater fishing in the U.K.
If you can get hold of them, do try them, they do make a difference.
If you are a bit handy Googling images "fishing boom rig" will give you heaps of ideas.