It's not easy to get an annulment in Canada. There are some extremely specific grounds which support a claim for an annulment. You could claim fraud in that she hid her large debt, but I suspect that this isn't enough. There's a decent summary of annulment issues here: http://www.canadiandivorcelaws.com/annulment/
In your case, I can appreciate why you want an annulment. You don't want anyone who is trying to collect from your wife to come after you. Whether they can will depend on who is that debtor is and whether you have any joint assets. It sounds like you don't anyway.
You're already legally separated, and have been since the day that it was clear to both of you that the "spousal relationship" was now over, although you remain legally married. Many people go years remaining married although they are separated, with a separation agreement and all issues settled. The divorce really is just a piece of paper saying you're free to remarry.
Don't tell her what you're doing before you decide what it is you are doing. Make some appointments with family law lawyers in London who will give you a free consult, and ask them what they recommend in your situation and what it would cost. An annulment might not be any more advantageous to you than a quick divorce.
Does that make sense as a starting point? Check that website I cited and get back to me if there's anything else about this to discuss. If I've answered you then I'd appreciate a positive service rating please. Ratings are how I get credit from the site for helping its customers.