You can 2-way sync BookOn calendar with Google Calendar:
Edit the staff profile, scroll down and sign in with your own Google Calendar account.
Note: A warning from Google will show up because takes time for the app to get approved. You can safely ignore the warning and proceed.
Once is all set you can also sync Outlook calendar with Google calendar as well and all your external bookings from Outlook will sync with Google calendar and Google calendar will sync with your BookOn admin panel booking slots making them unavailable