Toronto is cheaper. Let me just say it up front so the suspense dies. But the gap is smaller than the internet makes it sound, and where you live inside either city matters more than which city you picked.
Rent is the whole argument, really. In London, a room in a shared flat in Zone 2 or 3 costs more than most newcomers budget for, full stop. Zone 4 saves you money and hands you a commute that eats your evenings, plus a travelcard that costs more than you think. Toronto is gentler. Rooms in shared houses near the big campuses cost noticeably less, and you can actually walk to the subway instead of budgeting for a bus to a train to another train.
Transit is where London stings worst. The Tube is the best metro I have ever used and the most expensive one I have ever paid for. Contactless caps add up fast even with a student railcard. Toronto gives you the TTC: one flat monthly pass, subway plus streetcars plus buses. When people ask how much does it cost to live in Toronto as a student, the transit bill is the one predictable line on the page.
Food is where London fights back. Groceries are genuinely cheap if you cook, which surprises everyone. Toronto groceries run higher, especially dairy and produce in winter. Eating out destroys budgets in both cities equally. No winner there.
The boring truth about the cost of living for international students in either place: neither is cheap. Toronto is the safer bet on a tight budget. London is worth the premium only if the program or the career network is clearly better. Don't pick London for the money. Pick it for the reason.
For a proper study abroad city cost comparison, run your numbers by neighborhood, not by city average, because city averages lie. A study abroad cost of living calculator app, iOS or Android, can do the math if it has neighborhood-level data, and monthly living expenses for students in London versus Toronto look very different once you do. After you land, a city by city student expense tracker app keeps the spending honest. And if you want a human to argue about the numbers with you, the pre-departure concierge at studyabroadadvisor.fyi does exactly that.