Well that's not typically the case, at least not if you cancel after 24h or if you make changes first. What typically happens then is you get the avios used back but not the top-up cash - and have to put up a big fight to get what you're rightly due. Has caused a lot of frustration for a number of us:

http://www.flyertalk.com/forum/briti...sh-refund.html

http://www.flyertalk.com/forum/briti...h-booking.html