Here's the information from the Flying Club website regarding Avis:
"Miles required :
12,500 for a £50 voucher (or equivalent in US dollars).
To exchange your miles for a voucher, call your local Flying Club helpline, then contact Avis reservations to check availability and make a booking.
Reservations are subject to availability.
Blackout dates may apply.
Car rental vouchers are valid for 12 months."
I presume that Hertz is treated similarly, although I could not find on the website the conversion rate from Flying Club miles to Hertz vouchers.