Its about 100 for a sole trader, but for big companies it increases for how many you employ, but i did my insurance through ALLIED, and they give you a £50 cheque back if you join the fed, or are a member if you join within 3 months from starting your premium
Benefits are there shop, one of the biggest trade org's for window cleaners, professional image, stickers and letterheads etc....
More of a professional approach for commercial i think, probably not a big deal on domestic unless you are doing really high end stuff ?