My understanding is that when the FISO has said "...at your discretion..." he is just saying "I don't know of any reason why you shouldn't take off but I cannot clear you so do what you think is best". Your reply is
telling him what you're doing - so no "at my discretion".
You don't have to take off - you might have seen the formation of 15 Yaks that have just turned up on short final for a run and break - replying "Holding at my discretion, G-TC" would sound silly...
