A bit more searching online finds that the Statute covering roadsigns is the
'Traffic Signs Regulations and General Directions'.
From Wikipedia - The Traffic Signs Regulations and General Directions (commonly abbreviated to TSRGD) is the law that sets out the design and conditions of use of official traffic signs that can be lawfully placed on or near roads in England, Scotland, Wales, and the Isle of Man.
Another interesting link which explains what they are is
http://www.ticketfighter.co.uk/signregs.htm . Notice the bit that says '
A sign which is not contained in this manual or is not a permitted variant of a sign in this manual is illegal unless is has special approval from the Department for Transport.' Which basically means if a sign is not in the manual or approved then it can't be enforced.
Plenty more links if you search for them. Always worth checking the signs are correct if you get a ticket, a few minutes work could save you a fine or points.