Under the terms of the Disability Discrimination Act (DDA) 1995, treating a disabled person less favourably because of their disability is unlawful in the UK. Since October 1999, service providers (including information services) have had to consider making reasonable adjustments to the delivery of their services so that disabled people can use them - including the provision of an 'accessible' web site.
1 in 7 people in the UK have some kind of disability to a registrable degree. Given people's reticence to reveal information about themselves that might affect other people's view of them, that figure might well be higher in reality. 20% of the UK population also have some form of reading impairment - and that figure will include many people you already work or trade with every day. Bear in mind also that web use can be affected by a range of problems, including blindness, partial sight, dyslexia, deafness and hearing impediment, and physical impairments to control of arm and hand movement. The Disability Rights Commission estimates that people with disabilities in the UK represent a total of £50bn of spending power on goods and services every year.
The World Wide Web Commission published the Web Accessibility Initiative Guidelines in 1999 - already some 4 years after the DDA was published. To give some idea of how little progress has been made since, the Disability Rights Commission Report into web access and inclusion for disabled people in 2004 found that only 19% of websites met even the lowest priority of the WAI checkpoints.