I too remember them in the 50s and 60s. Think there was also a resurgence of them not many years ago, but think they were much smaller (surprise, surprise) and were intended for freezing them at home in bulk.
Think the triangular shape was probably a marketing USP - nothing else on the market like it at the time. But it was also very easy to hold and readily contained the iced juice, unlike a lolly which would melt, drip, drop off etc. So you could play, carry this handy drink/ice around with you and there was less mess.
Bought many Jubbly's from this place when it was "The Candy Store" in the 1950s/60s. (Junction of Tresham Road and Kingstanding Road). Then we'd sit on the wall in front of the shop and slurp our way through them. The wall seems to have been replaced by a fence. Pity, I remember my friends and I sitting there solving all the worlds problems. Viv.