Broadly correct – in the vast majority of cases BS, BS EN, ISO etc, are not legally mandated. However:
1. Courts love them. If one were to find oneself in court trying to defend why you hadn’t followed relevant ‘good industry practice’ then the benchmark that they often apply is equivalence to the relevant national/international standards. Basically, if you didn’t follow them, you’d better have prepared a very good reason why not, & this does not normally include “I know better than the people who wrote them”!
2. As has been said, under the soon-to-be-law construction Products Regulations it will be mandated upon certain suppliers within the fire protection industry that they must CE mark their products before placing them on the market. My understanding is that this is a EU Regulation that does not need to be translated into national regulations on a country-by-country basis (unlike the Construction Products Directive, which did, and the UK refused to fully implement it). This will mean that, effectively, compliance with these standards will become a legal requirement, for those who wish to place products that are within the scope of the CPR on the market, in the UK.