How do you mean a non-known brand? If your address is showing as live for FTTP then you approach other ISPs and place an order. If those ISPs don't offer FTTP then you can't have it from them.
BT are likely to be the cheapest option (especially once cashback, 'free' gift deals are taken into account), at least until Sky start offering FTTP.
Well until this morning I'd never heard of Zen or Cerberus so those to me would be a non-known brand.
So I purely have VM for it's BB. We only have the phone line because we have to - it's been unplugged for years and we just use our mobiles for any calls. Do companies like Zen/Cerberus provide the same type of deals or do you have to provide your own phone line (like from BT) then get the BB service on top?