Pilots with a valid Instrument Meteorological Conditions (IMC) Rating are recommended to add 200 ft to the minimum applicable DH/MDH, but with absolute minima of 500 ft for a precision approach and 600 ft for a non-precision approach. The UK IMC Rating may not be valid outside UK territorial airspace, therefore IMC Rated pilots should check the validity of their rating for the State in which they intend to fly. If the rating is not valid pilots must comply with the basic licence privileges, subject to the regulations of that
State.
Speaking with a lawyer's hat (or should it be wig?) on, I don't believe this is laying down mandatory minima. The entire paragraph reads as an advisory statement. The use of "but" links the second part of the sentence to the first by qualifying the general statement that IMC holders are recommended to add 200 feet etc, but they should always work to a recomended minimum of 500 for a precision approach etc.
The AIP does not itself lay down a mandatory minimum restriction on descent below a certain altitude - this is defined in the legislation which is the ANO, which prohibiting descent below the published minima. The AIP is where those published minima are to be found. Had there been a separate (different) restriction on IMC holders then the place to find it would be the ANO, not the AIP.
Taking the point a stage further, the penalty for a breaching the requirement in the ANO is a fine. The only offence created is breaching Article 49, which refers only to descent below the published minima for the particular airfield and approach. There are no separate published minima which apply to IMC holders and therefore no sanction for any breach of the DA + 200' etc.