In Ontario, the main airports are Toronto Pearson and Hamilton International.
I have found that the globespan flights, are very cheap - they fly into Hamilton (which is 40 mins from Toronto, but close to Niagara Falls).
I flew home to Scotland in May for $420 return that was direct to Glasgow and was including taxes.
The earlier you book ,the cheaper it is. generally, avoid Christmas/New Year, July and August. All other months are pretty reasonable with these airlines.
I would never fly to/from UK with any other airlines apart from these ones ever again (unless of emergancies that is)
Hope i was of help!