Keep looking for a cancellation - if you are lucky you might get one.
I had the same problem they did not let me take the exam and had to rebook bu got lucky with getting a canceled appointment.
Getting a date in Croydon is a pain - they dont seem to have any dates till 29 Jan but can see earlier dates for different test centres.
Last option for you to is think about changing the PEO date
https://www.gov.uk/ukvi-premium-service ... ppointment
You want to cancel or reschedule your appointment
You can cancel or reschedule your appointment using your online UK Visa and Immigration services account.
You must:
cancel at least 5 working days before your appointment
reschedule at least 48 hours before your appointment
You’ll be refunded the full fee for cancellations made at least 5 working days before your appointment.
You don’t need to pay the fees again if you reschedule your appointment.
hope it helps.