So time the train a minute later and people end up arriving a minute later, and so on as infinatum until trains don't ever leave.
Conditions of Travel quite clearly say you should be on the platform in reasonable time (I think it gives a figure, but my memory isn't what it was) beforehand.
I wasn't really suggesting changing the underlying Working Timetable, merely suggesting a way of deriving public advertised times from it that would allow a consistent expectation about the public time to be communicated to passengers. Essentially, take the working time and round the public time down by a maximum of a minute, as long as the difference accommodates the planned dispatch process as per these examples:
Working time of 08:25:30 would give a public time of 08:25:00
Working time of 08:25:00 would give a public time of 08:24:00
This way trains could operate as now, but passenger would always be able to board until the clock ticks over to the publicly advertised time.
Strangely, the Conditions of Travel
http://www.nationalrail.co.uk/Conditions%20of%20Travel%202016.pdf don't appear to have anything to say about this issue. Or at least I couldn't find any reference to it searching for several different words I thought might point to the relevant clause - I'll admit that I didn't read all the way through.
In any case, I would say that if the railway industry thought that the Conditions of Carriage were a suitable way of communicating expectations to the public, then we wouldn't be communicating at all. I haven't read Amazon's or any other internet retailer's terms and conditions despite ticking the box and I'm sure you have loads more interesting things to do than that too... If I stood outside King's Cross for an hour and asked 100 passengers and found one (non crank or member of staff) that had heard of the conditions of travel, let alone read them, I would be astonished.
Of course it would still be good advice to get to a platform in reasonable time, though what would be reasonable would presumably vary with train type. For example, I find that 10 seconds is ample time to get from ticket gates through the nearest train door before the chimes on my train to work at platform 2 of Welwyn Garden City, but would expect to get to an infrequent long distance service at a less familiar station up to 10 minutes before.
Additionally, being able to give passengers a consistent understanding of what the timetable means may even improve the safety of our industry's staff on stations. I would bet that the number of assaults on platform or gate-line staff that had their origin in a train missed by a few seconds in this manner will not be zero.