In my experience, "BSI" doesn't write it's own standards. The donkey-work tends to be done by 'volunteers', who sit on the committees managing the various standards. The constitution of these committees varies, but as regards fire standards it tends to be mostly people representing interested (and relevant) trade associations, with a sprinkling of CFOA, IFE, testing lab's, certification bodies & sometimes major users. If you belong to a trade association, you might find that they have/had a seat at the table. BSI just administers the process & checks that the committee doesn't stray outside its rules & its 'Standard for Standards'. In my experience it does little to influence the technical content.
The quality of the standard is crucially influenced by the make-up of the committee & who volunteers to do the drafting. If the committee is dominated by (say) one or more trade associations then they could either have a vested interest in driving the standard requirements low (making them easy to comply with) or, more likely, driving them high, so that the cost of kit & installations goes up, & they help exclude suppliers outside their Association. If it's a BS EN, then you have to add all the competing National interests as well!
Basically, the quality of the standards depend upon the constitution of the committees, & how seriously the members take their responsibilities. If you get crap representation, or if the members don't all pull their weight & put the necessary work in, you'll get a crap standard, or perhaps a biased one. We all get a chance to comment on them when they're in 'draft' form as well, but you do have to keep an eye on the BSI website.
Given the fact that most of the work is done by volunteers, I do wonder why they cost so much!!!?