Extract from The Air Navigation Order 2000, Schedule 8, Part B 1:
Instrument meteorological conditions rating (aeroplanes) shall within the United Kingdom:
(a) subject to paragraph (c), entitle the holder of a United Kingdom Private Pilot's Licence (Aeroplanes) or a United Kingdom Basic Commercial Pilot's Licence (Aeroplanes) to fly as pilot in command of an aeroplane without being subject to the restrictions contained respectively in paragraphs (2)(c) and (f) of the privileges of the United Kingdom Private Pilot's Licence (Aeroplanes) or (2)(b)(vii) or (ix) Of the privileges of the United Kingdom Basic Commercial Pilot's Licence (Aeroplanes)
(b) subject to paragraph (c), entitle the holder of a JAR-FCL Private Pilot Licence (Aeroplane) to fly as pilot in command of an aeroplane in Class D or E airspace in circumstances which require compliance with the Instrument Flight Rules.
(c) the holder shall not fly:
(i) on a special VFR flight in a control zone in a flight visibility of less than 3 km;
(ii) when the aeroplane is taking off or landing at any place if the flight visibility below cloud is less than 1800 metres.
Paragraphs (2)(c) and (f) of the UK PPL privileges which do not apply to the holder of an IMC rating:
(2)(c) He shall not, unless his licence includes an instrument rating (aeroplane) or an instrument meteorological conditions rating (aeroplanes), fly as pilot in command of such an aeroplane:
(i) on a flight outside controlled airspace when the flight visibility is less than 3km;
(ii) on a special VFR flight in a control zone in a flight visibility of less than 10km except on a route or in an aerodrome traffic zone notified for the purpose of this sub-paragraph; or
(iii) out of sight of the surface.
(2)(f) He shall not, unless his licence includes an instrument rating (aeroplanes) or an instrument meteorological conditions rating (aeroplanes), fly as pilot in command or co-pilot of such an aeroplane flying in Class D or E airspace in circumstances which require compliance with the Instrument Flight Rules.
The refernces to BCPL(A) privileges have exactly the same wording.
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So, you can fly as pilot in command of an aeroplane in Class D airspace in circumstances which require compliance with the Instrument Flight Rules (e.g. in IMC).
I would strongly suggest that you avoid taking any further instruction or advice from those incompetent instructors who were unable to quote this quite fundamental piece of legislation. The fact that you are blundering around the FIR, not knowing the privileges of the licence and ratings you hold is another matter. Ignorance of the law is not accepted by the courts as a valid defence.