My guess is it will come with a 500GB ssd as standard for £500. With a 1TB SSD option for an extra £100. So £600 for what should be considered a minimum acceptable amount of storage. Would then have to hope there will be a method of caching games from an external HDD to the internal SSD, so at least you can keep a library of games on disk so you don't have to be deleting and re-downloading that often.
Then there is the possible complication of expecting you average non-techie punter being expected to know what a M.2 SSD is, and how this is going to drive the design of the console. I mean that the designers are probably anticipating your average computer illiterate to the scared at the sight of what is basically a bare chip board, compared to an actual sata ssd or hdd, and will design the console without an option to upgrade with an m.2 at all, i.e. no extra slot, or worse no accessible slot, if you were actually planning on splashing out as high capacity m.2's come down in price over the life of the console. Hopefully they won't be that dumb.