Icelandair is showing cheap open-jaw flights to London & Dublin from Toronto for only $378 CAD Including taxes.
You can also add a stopover in Iceland for up to 3 days for an additional (approx) $65.
Select dates departing from January to March & end-April
HOW TO SEARCH FOR DATES AND BOOK:
1. Go to Google Flights to search for dates
Sample Search for London/Dublin open-jaw flights
Sample Search for London/Paris open-jaw flights
Sample Search for London/Amsterdam open-jaw flights
Sample Search for London/Copenhagen open-jaw flights
2. Use the following date combinations:
Jan 9 to 23
Jan 13 to 27
Jan 16 to 30
Jan 23 to feb 5
Feb 3 to 12
Feb 6 to 18
Feb 12 to 25
Feb 17 to 27
Feb 19 to Mar 3
Feb 27 to Mar 9
Mar 2 to 12
Mar 5 to 17
Mar 10 to 23
Mar 12 to 24
Mar 18 to 30
Apr 23 to 30
NOTE: These are not fixed dates. There are many more combinations available. Stick with Monday through Thursday dates to get the cheapest priced flights.
4. Google Flights will give you the option to book directly with Icelandair
Screenshot from Icelandair

HOW TO ADD A STOPOVER IN ICELAND FOR AN ADDITIONAL $65?
1. Go to Icelandair
2. Select the Icelandair stopover option
3. Search for:
Toronto to London
Paris, Amsterdam, Copenhagen or Dublin to Reykjavic
Reykjavic to Toronto
4. Use the dates provided above for guidance and simply add a date at the end (for up to 3 days). You’re only allowed a stopover in Iceland for up to 3 days.
Example: March 3 to 11 to 14
Screenshot from Icelandair
