You can't get an 8 day travelcard, only 7 (and 30, 31, 32... 278, 279, then 280-365 days costs the same).
I was surprised as well! I was issued 8 days for the price of 7.The gentleman at the office in LHR replied with a smile when I asked him.. He just asked when am I coming back to LHR gave him the date and YES 8 days for the price of 7!
Deposits can only be refunded 48 hours after a card is purchased, so if you really only need one tube journey on an LHR layover and don't have a usable contactless card, it would be cheaper to buy a single ticket (at least, if you don't trust yourself to remember to bring the card on your next visit).