Hi Dave,
If the gum in Great Britan was the flat cubed gum like it was in the States back in the Fifties, you wont find any. Canel Mexican gum works and Bazooka works in some of the fifties U.S. machines.
Here's a site of Double Bubble from Groovy Candies.
http://www.groovycandies.com/pc/viewCategories.asp?idCategory=527P.S. What does the "D" stand for after the 1 on your British machines. We have "C" for cents.
I remember the old British pennies from that era. Wern't they similar in size to our half dollar?