Toronto's a big city -depends on where about you are travelling as well. If you fly into Toronto airport, you have to go through the city to get to downtown - Hamilton is on the 'right side'. Worked well for us last year. And if you want to go to Niagara, flying into Hamilton over Toronto saves you time on that trip.
Gatwick appears as a London airport on most holiday booking sites - how far is that to London City centre?