You can cancel British Airways if you go into your account or their website by clicking the "Manage My Booking" button. Rebooking gets a full refund if canceled in 24 hours of making the booking. A charge will most surely be added in case it passes to 24 hours after making the booking, but this may vary depending on the type of fare-you could get until a certain amount of time to cancel and get a refund, but others not at all.
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