With regard to the second para. - your first statement is correct, but your conclusion does not follow. BAA are actually exempt from the Building Regulations, but there is absolutely nothing stopping them demanding contractually that their suppliers conform with as much or as little of the relevant guidance documents as they wish. They can demand compliance with French, German, Zimbabwean or Outer Mongolian guidance and/or regulation if they want to, and whilst you or I might have views on how sensible it is, if that's what they want then any suppliers, tenants etc just have to like it or lump it! If their suppliers don't comply, they'll be in breach of contract.
As another example, London Underground are exempt from the requirements of the Building Act, but use Class '0' in some of their materials fire performance requirements, for installations that are clearly not within the scope of 'linings' as defined in the ADB. They also demand extraordinary high levels of smoke and toxicity performance too. If your client were to be setting up shop in one of their stations, any bleating about the ADB being inappropriate / inapplicable would receive a very short, succinct and definitive answer!
I'm afraid that your client just has to do the necessary and comply. I haven't tried, but I imagine a quick trawl of the Internet would reveal suppliers of Class '0' fabrics or fabric-type materials.
Your first point wee brian was raised earlier in the discussion by Kurnal your second I agree with if I remember correctly polystyrene ceiling tiles disappear so fast that under test they meet the Class O criteria.
Fishy As I understand Class O is defined only in Building Regs and does nor appear in any BSS - check out ADB vol 2 Page 120 Para 13 - so I would reason that outside the building regs it does not exist. Furniture and Fittings are not controlled by Building Regs check out ADB vol 2 page 63 B2 iv unless you consider portable point-of-sale displays are not furniture and fittings?