@Sarah1120
Sarah, the dates are in fact blocked, this is Airbnb's way of punishing you for cancelling a confirmed guest reservation. They want the world to know you canceled a booking!
It's petty, of course it's petty, and if you get onto CX they will fix this for you and allow you to block those dates.
You have to understand the platform is automated and responds to certain protocols! It senses you are a 'bad' host because you have cancelled a guest reservation, and is not influenced in any way by circumstances outside of programmed EC's .
As you are in the UK try ringing this number and asking for their help..... +44 203 318 1111
Cheers.........Rob