Spread betting is indeed considered gambling in the UK, despite the similarity to options trading, and not just by the tax folks at HMRC. The financial betting is just another link at a typical website like Inside Poker where you can find this explanation of spread betting.
To guess at an answer since there may not actually be an official one (and I would bet it is a pretty well hopeless exercise to phone the CRA where you would likely spend a long time getting the service rep to actually understand what the heck you were talking about), I refer first to this explanation by accountant Tim Cestnick on the taxation in Canada of pure gambling like poker. The essence of it is that anything, including poker, can be taxable if it is a business. (I especially love the classic extreme example of this in the fact that bank robbers are liable to pay income tax on what they steal).
I suspect the CRA would apply the same principle to any winnings on UK-based spread betting. It notably does not matter that the UK government doesn't consider such winnings taxable. If you are a Canadian resident for tax purposes (and you pretty well have to have left the country and burned all your bridges in Canada to avoid that) then you are subject to pay Canadian tax on your worldwide income. The fact that the UK doesn't tax spread betting makes it simpler in principle for Canada to claim tax since there are no complications from having to avoid double taxation and having to refer to the rules of the Canada-UK Taxation Convention to decide who would get to claim your taxes.
But you have to report the income to the CRA. Making it much more difficult for the CRA to actually track you down should you decide to hide such income, were it actually a business, are some other provisions of the above Convention, which is further charmingly sub-titled "For the Avoidance of Double Taxation and the Prevention of Fiscal Evasion With Respect to Taxes on Income and Capital Gains". Way down in Article 24, it says in effect, that the tax authorities will exchange what information they normally collect and since the UK HMRC have nothing to do with taxing spread betting, it would be a surprise if they had any data about foreigners doing spread betting to send to the CRA, should they ever even ask HMRC. Nevertheless, it is better to be honest isn't it?
btw, if you are able to successfully tell whether the market is going up or down and make any money spread betting, then I would sure like to know what your bets are. ... Naw ... I think I'll just stick with the boring plodding diversified portfolio, which is unfortunately taxable all the time somewhere.