Enjoy!
http://www.legislation.gov.uk/uksi/1.../contents/made
Actually the bit you want is
here and basicly says to be road legal it must be
"clearly and indelibly marked “BS AU 193/T2”, “BS AU 193a: 1990/T2” or “BS AU 193a: 1990/T3” (being marks referred to in the British Standard Specifications indicating that certain requirements specified in those instruments are met)."
Some more details regarding exhaust markings
here