The apartment itself is completely fine, however it is not, I repeat, explicitly not allowed in the entire building to have any flat be used for shortstay or Airbnb purposes. Since Cahit also forces you to enter via the concierge it's up to you, lie and maybe enter the apartment if the concierge doesn't really care or be left without a place sleep once your in London. I absolute do not recommend staying at his place, safe your trouble and money, you'll be the one that's out of luck if this facade doesn't work out the way it apparently usually does for him.