Branches that have vouchers for 20% off Nando's (till 31/12/2014) are:
Free 1/4 chicken at North Harrow with 6GBP min spend
Also 20% off Subway at Ealing
I only checked The Gym branches within London. Some have deals for Frankie & Benny's and other places. Those with multi-site access like me can get member vouchers for any The Gym branch!
Check what offers your local 'The Gym' has under the members vouchers section of 'my account'.