The BWF from my dealings with them through my father and through one of my employers, seems to be a good fit for medium and large businesses...
In terms of customers appreciating the BWF membership, remember that they're involved in developing standards and running things like cetification schemes (such as their Certifire scheme) not to mention vetting the financial health of their members, all of which are much more likely to be of interest to a specifier/architect than they are to Joe Public who just wants to make a plain english request and have you do it to meet building regs.
They do a lot of good things and I wouldn't dismiss them out of hand, but you need to see if the services and benefits they offer to members are going to suit your business... There's
this calculator, you could also ring them and be as blunt as to ask "I'm considering joining the BWF, what would you do for me as a member that makes it worthwhile"
In fact, a lot of it seems to be biased towards volume manufacturing... Chain of Custody, CE marking, U-value simulation are all things which impact positively on the saleability of specific products to an installer/end user, but would be much less valuable if you're primarily making and installing bespoke joinery.
In conclusion, they may or may not be good for you, depending what you make, and where you want to take your business.