if you don't want to use car rental, your options for getting from Toronto Airport to London ON are the train or bus. You can take the via rail train or greyhound bus services directly to London or you can use a combo of bus and train. In this case, take the TTC Downtown Express from downtown Toronto to Union station and from there the train to London ON.
It's not a super long trip between the two cities, so I imagine it wouldn't take a ton of time traveling by public transport. I just had lunch in London the other day on the way to Niagara Falls. It's really getting built up. Sure, it's no Toronto but there's tons to do there. Is that indoor market still there? I loved it so much when I visited years ago. Still wear the earrings I bought there.
you mean the Covent Garden market, right? if so, it is still there for sure. I prefer personally the Western Fair market. I think iit has better choice of goods at better prices. Covent Garden is a bit of a rip off...I guess cos it's kinda a copy of the Covent garden in London England.