Canadians pay tax on worldwide income. Generally the tax treaty is for when you have already paid tax in the other country. My guess is that you will have to pay tax, but its worth a try to find out.
You say that the investment is not taxable in the UK.
The easiest way to get an answer to call Revenue Canada. Your call is anonymous and you can also choose to not have them record the call.
The first level people are not up to complex questions but they are required to pass you along to more senior people when they don't know the answer. The key is in how you ask the question.
e.g. "I would like to ask a question on the treatment of foreign bonds of a tax-free nature. Could you pass me to someone knowledgeable in this area, please?"
If they try to pawn off a made-up answer, press with "Could you please give me the specific article of the Tax Act you are referring to and any Information Circulars that pertain?"
They do have a website and the search engine is pretty good, so you can start with that.
Another route is claim $0 and write a note on the interest income line and then attach a letter to your return, explaining that your understanding is they are not taxable hence your claim of $0 and a copy of the Saving Certificate - or page from the website showing the Tax-free nature.
Of course you may know of a British taxpayer who would like to buy them from you. You'll be subject to any capital gains on the sale.