A bit late but.....
A second Building Control Surveyors perspective (Please bear in mind without the full details of the scheme this is only an interpretation on the :
In the seminars held by DCLG on the 2006 version of the ADB, there were comments to the effect that additional measures had been placed on resi care due to a high amount of mismanagement and unrealistic escape strategies due to low carer/patient ratios. The BRE research was mentioned in the seminar discussing the two bed scenario and the fact that potentially where two single beds were in a room sprinklers would do very little to save the person in a bed if it was the source of the fire, but potentially a person in a second bed in the same room could be protected. I believe dead pigs in pyjamas were used for the research!!
This was there approach to the situation and many BCO's/AI's attended the seminars so I assume this may be the reasoning for the response you have received.
I think following the guidance in the ADB the BCO is justified in the approach he has put forward, but I would add that in my opinion sprinklers would only be required to the new areas (obviously this would be dependant on suitable compartmentation arrangements/building layouts). There is no increased risk to the existing areas of the building, because the sprinklers are being installed to enhance the life safety of the new areas.
Secondly I would also add that the ADB is only guidance as others have mentioned, if an alternative strategy can be provided that addresses the risks, then why should it not be potentially be accepted.
Thirdly a note to Kurnal to say that whilst there are obvious delicate issues involved in dealing with an application that has already been submitted elsewhere, if an alternative Building Control Body accepts a solution, they would hopefully be doing so because they are satisfied with the proposals and could justify their approach. Just because one inspector does not accept a proposal, it does not mean it is an unsatisfcatory solution, or that it is wrong for others to accept it if they have a different opinion. Legally as long as the works have not commenced a client is free to choose alternative providers.
Finally changing the double bed post completion for two single beds would definitely not be a material change of use (these involve chnages in the buildings use or an increase in the number of bedrooms, but it could be perceived as a "material alteration" in that the means of escape will have a lesser level of compliance following the alteration, but as Kurnal says, it is rocky ground on whether Building Regulations can be applied to furnishings, and I would be interested to see the outcome in court!!