Free "creative" haircut at the Vidal Sasson Academy
(or £3 for classic haircut, men or women)
Call up to make an appointment and bring the voucher.
The Sassoon Academy is a hairdressing school in London's Mayfair (they're moving to Westminster in June 2017), they usually charge £12 for a haircut by the trainee hairdressers (or £5 if you're a student), but they're currently having a promotion - FREE for a "creative" haircut, and £3 for a "classic" haircut.
A "classic" haircut is just your typical haircut - you let the trainee stylist know how you'd like your hair to look. Go for the "creative" haircut if you want a statement look which your stylist (usually on an advanced cutting course) will propose.
I used to frequent the Sassoon academy as a uni student, and still go back for a haircut now and then when I have a spare afternoon. I've always had good experiences with the stylists and teachers, but of course it's riskier having a trainee cut your hair (even if there is a teacher on hand to fix it) and it does take a large chunk of time, so if you are going then be prepared to dedicate around 3 hours. Totally worth it in my opinion though - a level 1 Sassoon haircut will set you back around £70, and because there's always a teacher around checking their work the students are generally quite diligent and careful. I've had some of the best haircuts in my life here!
The Sassoon academy is in London only, if you're not in London look out for community colleges near you which run hairdressing courses - they usually have a good prices on haircuts.