You might get some better advice over on
PePiPoo where there are more people who like the legal side of things.
From what I understand a road with street lamps (more than 3, spaced correctly, etc.) has a 30 limit on by default.
Also, speed limits must be signed correctly.
But once you are inside the street lighted area there is the assumption that you have worked out that it's a "built-up area" so signage problems at the start of the area won't help you. Or that is what I understand from reading a post on PePiPoo's forums from someone who seems to know what they are talking about.
It may well depend on which law/act they have charged you with. Or it may not.
I would be interested to find out what the British Standard mentioned in the law contains, and if the lamps meet that standard.