It has been a while since I had a UKAS assessment, but when I did all observations and non-compliances included a reference back to the ISO doc, UKAS RG8 or HSE guidance that related to the non-compliance – this is really useful if you are challenging the assesment and at least provides the basis on which you can challenege it – no one i perfect and everyone makes mistakes from time to time! Could you post what the reference included in the non-compliance was?
My understanding is that if you cannot access but can see the item/ACM then a material score is appropriate – using the surveyors experience of the most likely product and asbestos type for what they see before them, e.g. a high level AC panel etc and what condition it is in as the surveyor can see it.
Where the surveyor physically cannot see any ACM because the area or item is inaccessible (room or duct etc) then, as you say, a presumption of asbestos shoudl be made, but a material assessment cannot be conducted. The surveyor should record that the room or area should not be entered until an inspection by a competent person has been conducted and that it is safe (in regard to asbestos) to enter.
In either case the inaccessible area/room should be recorded as ‘presumed’ asbestos and noted as inaccessible in the asbestos register, on the plans and in other appropriate parts of the report as if it had asbestos identified pending further investigation.
In regard to this particular instance you might refer your assessor to HSG264 (second edition) and the examples given on pages 43 and 44 where inaccessible areas are shown but there is no material assessment provided much like you have described what you do.
With best regards