The 'FreeStationWifi' at NR-managed stations is always available and fast, in my experience. Probably the best around.
Greater Anglia usually works well but can be a bit patchy in coverage (eg. at Ely, for one example).
Still, better than a major railway station that apparently has no wifi at all (looking at you, Nottingham, given I was there just a few hours ago; wifi would have been rather useful to work out why various trains were delayed and so how to most quickly get to where I was going).