Do not book. I have had a terrible experience with Kannell. My fiance and I arrived at check-in time (3 PM), which we had notified the host in advance. We had been given the access code to the front door the night before, but were told that the code for the bedroom would be given just after 3. We waited a few minutes to receive the code but heard nothing, so I messaged Kannell letting him know we were here and if he could please send the code. He did not respond. We sat in the stairwell and waited. I messaged again at 3:30, no response. I called twice, no answer. We ended up leaving around 3:45 (time goes really slow when you are sitting in a hallway with all your luggage. After an hour, I reached out to Airbnb and let them know we couldn't reach the host. It wasn't until an hour and a half later that he finally responded, only to then claim that it was my fault because my email must have been wrong (which is definitely not the case). We requested our money back and of course he refused it. After lots of back and forth between me, Airbnb and the host, Airbnb has now refunded me. Whether or not the code was not sent due to automated system errors or the host, we still had to wait an hour and a half with no communication and unsure as to whether we would have a place to stay.
Do not take the risk of this happening to you. Arriving in London and having to worry about where you're going to stay in the middle of the day, considering both travel time, prior plans, and costs, is not worth the price!! We found a hotel nearby on booking.com for nearly the same price and had no issues.