The IOV have a trade fair in the autumn, at Stonleigh, near Coventry. Where a lot of manufacturers and distributors attend. I've had dealings with them, through holding seminars etc. and found them sensible, serious and reliable. I'm not a member, mainly because I do 90% broadcast and the IOV isn't active in that field.
They seem to be aimed at small businesses, wedding videographers, industrial stuff, and semi-pros. I think it's a case of using the IOV to raise your own standards, not a case of having to be "good enough" to get in.

I think that if you're going to start charging for video services, it doesn't do any harm to belong to a trade organisation like the IOV but it's not a short-cut to getting clients, more a chance to talk to others at your level, compare notes and learn from others.