Is there a bead seat diameter listed on that tire? In England, 28x1 1/2" tires have a 635mm bead seat diameter. In Canada, 28x1 1/2" tires have a 622mm BSD. An English bike made in Canada could be either, I guess. Look closely at the tire and see if either of those numbers come up. If not, hold it up next to a modern "700" wheel. If the rim is the same diameter then it's the smaller size, which is kinda nice cause most modern hybrid bikes use that size and it's easy to get. The bigger ones aren't hard to get but the selection isn't as nice.
Never seen a 28" camel back myself. Very interesting. What's the seat tube length?