Thought this information might useful for some, my grandmother had glaucoma so I get eye tests for free and there may be other reasons for you to get it for free that people didn't know about.
Here's the article: http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=895#
And the details are:
You are entitled to a free NHS sight test if you are:
* under 16 years of age, or 16, 17, or 18 years of age and in full time education,
* 60 years of age, or over,
* registered as blind, or partially sighted,
* diagnosed with diabetes, or glaucoma,
* 40 years of age, or over, and have a first degree relative, such as father, mothers, sister, or brother, with glaucoma, or you have been advised by an opthalmologist (a doctor who specialises in eye conditions) that you are at risk of glaucoma, or
* you are receiving certain benefits, or credits (see 'further information' below for more details).