The Guides: It would seem to me that these can be compared with the 'Highway Code' - it's not a legal document but if you don't heed its recommendations that can be taken into account if you are before the courts. The Guides are like the 'Fire Safety - an employer's guide' and compliance with that, to quote page ii: "you will normally be doing enough to comply with the law".
Re BSI Committees: I sat on one for a few years in the 1990s. Very broad spectrum of representation - Government, manufacturers, testers and users. So while current best practice is a large part of the equation, many parties also have financial considerations. These affect the final Standard in many ways. Equipment cannot be too expensive or complicated to make; it needs to be reasonably easy to test for compliance; it should not be too expensive to buy or maintain. So a balance has to be struck between what is desirable and what is practicable. And in addition we have to take account of the European and other International standards. So BSs tend to be pragmatic in nature and may not always represent the limit of current 'Best Practice', particularly if several years old.