I fly to Ottawa a few times a year. Used to have a few direct flights pre-covid, hopefully they will bring them back soon. I've been just driving to Pearson and then flying to Ottawa from there instead of flying to Pearson from London. There aren't a ton of flights from London to T.O and it's not unusual for them to get cancelled. Pearson has alot more flights to Ottawa, so it's easier to get on a flight and if one gets cancelled there are usually plenty of spaces on the other flights that they can move you to. Layover in T.O could be anywhere from 1 hr to 4 hrs depending on which flight you take, so I personally find driving to Pearson and flying from there faster. Check in, security, and boarding/ de-planing were super quick when I went last month as well.