You really want to run the gauntlet and try this? As CWS states the cancellation is automatic and it is the same for all airlines - not just BA. You will get taken completely off the system and additionally if you are on a very busy flight then a waitlisted passenger will get confirmed with your seat. If you want to risk it fine but this is one of the almost absolutes of air travel - sectors are issued to be travelled in the order stated only unless they are all fully flexible fares (and even then there can be some restriction!)

I would not try it to be honest....

FD.