I understood that you were not planning on using the ticket between LHR and CDG., meaning, you'd just throw away that coupon.

By going to BA and telling them that you do not intend to travel any further you might find yourself in a position where they would require a fee from you or even worse, claim you'd owe them a different rate alltogether...

In order to avoid this, you would just call BA and tell them that because of unexpected circumstances, you'd have to cancel your flight that day and would call them back when you'd know more. This way, you free your seat and you're off any further obligation.