I don't think so. Based on my experience, the BRC auditor always refer to existing regulatory standards. Determining if the ingredient is "acceptable" or not, based upon risk is not enough to ensure safe usage of the product. The company shall always refer to safety and legislative requirements since it is law of the country/land that you should be following and not merely the supplier specifications.
That's why we do internal verification of the raw material (i.e. purity testing) and refer these results to existing regulatory requirements to ensure that both the company and supplier are compliant with the standard.
Based on my past BRC experience, I agree with Marshall. Granted, I can't say what happens in your country as opposed to the US. We do some exporting of products, and we have to provide different tests for certain things as they are compulsory based on regulations in those countries.
During our audits, we have never had to provide regulatory specs; unless for higher risk products where regulatory specs are known. If you're working with dairy or meats, I could see having to provide regulatory specs over supplier specs. However, I work in a sector of the industry where supplier specs are valid for BRC.
I'd be inclined to think that ascorbic acid would fit into the category where supplier specs would be valid for BRC. We use ascorbic acid infrequently, but is listed in our ingredients and never came across the question of whether it meets regulatory specifications. I know that the FDA in the US does have a list of ingredients that are considered "Generally Recognized as Safe" (or GRAS). The FDA has a list of certain ingredients (food additives permitted for direct addition to food for human consumption) with statements of the percentage of the ingredient allowed in a finished good as well and other requirements -- such as purity. There's also a basic definition for what a standardized finished product should contain to meet the standard of identity of the product.
The FDA had the following about ascorbic acid:
§182.3013 Ascorbic acid.
(a) Product. Ascorbic acid.
(b) Conditions of use. This substance is generally recognized as safe when used in accordance with good manufacturing practice.