So if trains are sitting in platforms for ages, what reasoning can they possibly give for only boarding 5 mins before?
Perhaps I applied too much logic to the situation!
There are slightly more constraints than that, for instance some of the platforms (principally 9 and 10) are short and can't be used by Pendolinos at all, and 11 car Pendolinos don't fit the other short ones (2 I think plus 5, 6, 9, 10 and 13, 14). And the Cally can only use 1 or 12 if I recall as these are the very long ones.
It mostly seems to be that they prefer to fully clean and service the train before calling it. If they'd just bring it in, tip it out, lock the doors bar the crew ones and call it, people wouldn't all go down at once (if you have a reservation why go and stand by a train for half an hour when you can go for a wee and get your M&S banquet in in that time?). Similarly with WMT, if (aside from the Crewe) there were always two Birminghams shown as available* at once when they're physically there, which they often are, people would go to them in dribs and drabs (and some might choose the later one for better pick of the seats).
It was also mentioned above that WMT services aren't generally called until the platform staff are present, so one sensible change, if that is a safety/security requirement**, would be to have one TOC dispatch for the whole station regardless of whose trains it is, and have a dispatcher permanently allocated to each platform group so they are always there unless they've briefly popped away for the loo or something.
* I don't think they should put "boarding" actually up until the doors are open, but putting the platform number up would be fine.
** I'm not sure it is because nobody stops you wandering onto platforms when no train is going to be going from there any time soon - if it was, the gates would be more secure and they'd be closed until opened for boarding, which they're not - the default state is open and unstaffed.