G.Fast will be a long while to wait probably not available for us all until 2020. Why wait that long? Surely 100/20 would be nice.
G.Fast trials are due to end this summer, probably less than 6months away. I expect them to begin the roll out very shortly after, perhaps even before the end of this year.
How fast the roll out begins to benefit people will very much depend on how Openreach decide to do it. They may chose to install it in cabinets that will see the most improvement first, such as those in very densely populated areas before considering a FTTrN type of solution.
Time will tell. For now, I can't believe you NEED more than 80/20 on a consistent basis.