No idea how good M&S drones are (!) however via the Sparks app they are half price.
Looks like they have 3 available - 2 at £39.50 (now £19.75) and one at £25 (now £12.50)
Online and in store
Thanks Teamfortune for the extra info - You have to login to you M&S sparks account and activate the offer before attempting to purchase online, The instore purchase should be discounted by 50% when they swipe your sparks card and sub total. Hope this helps.