IMC exam
 

Since the ANO now says:

Instrument meteorological conditions rating (aeroplanes) An instrument meteorological conditions rating (aeroplanes) entitles the holder of the licence to act as pilot in command or co-pilot of an aeroplane flying under the Instrument Flight Rules except— (a) in Class A airspace; or (b) when the aeroplane is taking off or landing at any place if the flight visibility below cloud is less than 1,500 metres.

Hypothetically, if there was a question on the privileges of the IMC rating, (as i understand it most of the text books run sample questions on the privileges), if the "correct" answer expected in exams is the previous 1800m I take it is OK to accept 1500m as the correct answer or if there is not 1500m as an option to disregard the question altogether as not having a correct answer and therefore invalid.

I'd ask the invigilator at the time of the exam to either strike the question or provide a rationale for why you have to answer "incorrectly" to get a correct answer.

Perhaps you should read Standards Doc 11 paras 4.3.1 and 4.3.2.