Be aware it's free if you then buy glasses, £10 otherwise.
From Boots Website:
Our eye test offer helps you save money on our usual price of £25. With the voucher your eye test will cost only £10, or will be free when you purchase any pair of glasses of prescription sunglasses. You can also save £20 when you spend £90 on any pair of glasses or prescription sunglasses with our voucher. And, don’t forget, all additional pairs are half price too!
To take advantage of our great offers, simply print or download the vouchers and bring them along to your appointment.