Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from London, Ontario to Brussels as you normally would.
London (YXU) is the airport you will fly out of if you book a flight from London to Brussels. You should not have trouble finding flights to Brussels from London; the airport handles an average of 0 outbound flights to Brussels per day. You should also have plenty of airline options to choose from since 0 airlines have flights from the London airport to Brussels. It is only a 8.6 km journey to London (YXU) from London’s city centre.
This is defined as London Airport (YXU) and Brussels Bruxelles-National Airport (BRU). Codes like Brussels Bruxelles-National Airport are often used to just call airports uniquely. Charleroi Brussels Airport (CRL), Antwerp Deurne Airport (ANR), Ostende-Bruges Airport (OST), Liège Bierset Airport (LGG) are other airports near this flight route and their unique identifiers/IATA codes.