Just consulted HC this is what it states
113
You MUST
* ensure all sidelights and rear registration plate lights are lit between sunset and sunrise
* use headlights at night, except on a road which has lit street lighting. These roads are generally restricted to a speed limit of 30 mph (48 km/h) unless otherwise specified
* use headlights when visibility is seriously reduced (see Rule 226)
Yes looking at the Road Vehicle Lighting Regulations 1989 there would seem to be room for manoeuvre here (there doesn't seem to have been any amendment made to the original stipulations on the use of headlamps - although without the actual paper in front of me I find it difficult to wade through the various sub-paragraphs and amendments as I hate reading from a screen).
http://www.legislation.gov.uk/uksi/1989/1796/regulation/25/made
There may be additional factors not made clear by Big A's sister to him - in any case it would be helpful to know what the Constable noted on the penalty notice [/highlight]as being the reason for its issue.
"obligatory lights not working" WPC issued the ticket

That could have been a bit more specific in my view
Light or lights and which one of the obligatory lights?Irrespective of what is considered to be an obligatory light and in what circumstances it must be used, the following stipulations would seem to be quite specific;
Maintenance of lamps, reflectors, rear markings and devices
23.—(1) No person shall use, or cause or permit to be used, on a road a vehicle unless every lamp, reflector, rear marking and device to which this paragraph applies is in good working order and, in the case of a lamp, clean.
(2) Save as provided in paragraph (3), paragraph (1) applies to–
(a)every– .
(i)front position lamp, .
(ii)rear position lamp, .
(iii)
headlamp, .
(iv)rear registration plate lamp, .
(v)side marker lamp, .
(vi)end-outline marker lamp, .
(vii)rear fog lamp//..........
From:http://www.legislation.gov.uk/uksi/1989/1796/regulation/23/made
Although interestingly, from the same Regulation;
(3) Paragraph (2) does not apply to–
c)a defective lamp, reflector, dim-dip device or headlamp levelling device on a vehicle in use on a road between sunrise and sunset,
if any such lamp, reflector or device became defective during the journey which is in progress or if arrangements have been made to remedy the defect with all reasonable expedition;
So there may indeed be room to manoeuvre.
It seems to me that this matter is not entirely straightforward and, providing there has been no aggro directed towards the detecting officer by either the driver of the vehicle or indeed any of the passengers, then I would be inclined to contact the officer concerned and ask for a further explanation of precisely why the notice was given - in what after all seems to me to be a case where advice, rather than sanction, would have been the better way forward.