I just did a search for economy tix YYZ-LHR return. These are the conditions if I select 'lowest fare' or 'flexible' respectively on the search page.





And here are examples for First YVR-LHR return, in this case Flex/Discounted and Flex/Flex respectively:



If you're looking at tickets involving connections, you may instead of the above get a link to the full fare conditions - that lack of clarity is down to BA's IT not being able to pull out the 'simple' rules from a more complex itinerary.