The UK government accessibility guidelines have a significant overlap with the Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C). This defines three accessibility levels for Web sites: level-A (the easiest), Double-A (attainable by many sites), and Triple-A (difficult). Broadly speaking, the government guidelines demand that UK government sites reach the 'A' standard, with a few elements of the 'AA' standard mentioned below, plus a couple of extras covered in Other First Half Elements and Guide to the Second Half.
The government document repeats (sometimes three times) the 'A' standard rules of the WAI. They also exist as a complete subsection (188.8.131.52).
All I'll do here is match up the UK government guidelines with WAI material.
Use HTML to structure the document, not style it
Use Cascading Style Sheets to format and style basic elements of a website
The equivalent WAI rule is 3.3: "Use style sheets to control layout and presentation."
Any font sizes defined in the Cascading Style Sheet must be customisable by the end user - do not hard code
The equivalent WAI rule is 3.4: "Use relative rather than absolute units in markup language attribute values and style sheet property values."
Basically, use percentage values for font sizes, or possibly ems, though they can be tricky to harness.
Other forms of navigation should be available for users who cannot use pointing devices
The equivalent WAI rules are 9.2 "Ensure that any element that has its own interface can be operated in a device-independent manner," and 9.3 "For scripts, specify logical event handlers rather than device-dependent event handlers."
Tables - make line-by-line reading sensible
The equivalent WAI rule is 5.3 "Do not use tables for layout unless the table makes sense when linearized. Otherwise, if the table does not make sense, provide an alternative equivalent (which may be a linearized version)."
Linearization means how the material inside a page appears when all the table code and CSS is removed.
HTML page should validate against specified version of HTML
The equivalent WAI rule is 3.2 Create documents that validate to published formal grammars.
Note: the government guidelines are rarely used in practice. See UK Accessibility for more details.
This Tinhat page is valid XHTML to WAI Level-A standard
UK accessibility intro + menu
UK accessibility site reviews
Comment on DRC Report
Simple WAI level A checklist
Level A -
TinHat Level A+
Tips for web editors
Non-HTML files (PDFs) and accessibility
Gov guidelines for UK gov sites (intro)