I've just come back to this thread since I placed its eighth post, which describes the only time I've had cause to complain about early running, and the successful outcome of that complaint. I've been slightly surprised by the extent of subsequent debate, so I've done a bit of arithmetic to try to put the discussion into context.
In very broad terms, I estimate that I've caught around 2,500 trains and travelled around half a million miles by rail in the UK over the past 10 years. This travel has occurred in almost every part of the network, but (if it makes any difference) only very occasionally in the South West of England and only once in Wales. I'm not especially obsessive about being at stations well ahead of time but neither do I cut it fine. I'd like to see myself as pretty normal in that respect.
So only once (1/2500 journeys or 0.04%) have I been affected by this this problem. The incident occurred on a high frequency route at a busy time of day, so noone would expect to turn up more than two or three minutes ahead of timetable. To further reduce significance, I complained and the TOC responded appropriately.
It doesn't matter how many regular travellers post here saying anecdotally that they haven't had problems when they are habitually turning up with plenty of time. They probably wouldn't notice if the train that they easily caught was early anyway.
I have never since BR days, missed a train because it left early but there are those in situations where, through no fault of their own arrive at the platform in time only to see their train leave before its advertised time. In some of those circumstances, it may cause them to wait more than the 15-20 minutes that I and most home counties travellers expect to wait for the next train.
It may though be much worse than that, i.e. missing a connection that could cost a lot more than time. Rare as those circumstances maybe, they are still very real whatever delay and compensation payments might be offered as 'goodwill gestures' (for the operator's error?).
A-Driver has suggested that 3 minutes before departure is reasonable time to arrive* and I think that would still allow travellers to catch the majority of early departures. The problem is that although as a driver, his advice is well-intended and probably about right, such advice is never included in the ticket conditions. Why are the TOCs so reluctant to inform their customers?, - the fact that they don't could one day cost them dearly.
* Arriving at a platform at a specific time is not guaranteed however well planned the previous part of the journey is (unless you work in an on-platform kiosk

). There are still opportunities to take longer than is available to get from the entrance to the station to the correct platform:
the hardened eco-traveller at the only manned window, negotiating a multiple split-ticket trip to Lime St. via Doncaster, Blackburn and Burscough on a Senior Railcard
the person at the gate with a pushchair the size of a Ford Transit fumbling to find a ticket in a handbag with more nooks and crannys than an ants' nest
...and those two are without any help from officialdom.