This makes the huge assumption that pricing will stay as it is, over the coming years, in a rapidly changing market.
My FTTP ISP has an entry level 50Mbps/50Mbps product for £20/month which is only a little more than the cheapest ADSL offering. I don't see why Openreach won't follow suit especially as they are supposedly overbuilding existing FTTP networks and need to stay competitive, given that Virgin covers 50%+ of the population and have their own gigabit offering.