For this to work each platform would have to be individually barriered. Imagine the queues if there was only one or two gates per platform and 500+ people all had to pass it to get to the train.
Unfortunately it just wouldn't be practical.
Interestingly, there are occasional full ticket inspections at Kings Cross, but rarely and they're very labour intensive. East Coast carries about 50,000 passengers a day - not dissimilar to the other major operators - and the general principle is that it is the passenger's responsibility to choose the correct train to board corresponding with their ticket, while the companies operate regular or random ticket inspections during during journeys as they see fit. A radical change in which 50,000 tickets and trains a day are also confirmed before boarding isn't realistic.
As I mentioned in my first post, there will have been 2 on-board announcements before departure from Kings Cross which specifically identify "tickets marked Grand Central" or "GC only" which went on to confirm that "passengers holding tickets marked Grand Central or GC only will be requred to buy a new ticket". Perhaps you missed those announcments, but if you do 'appeal', that fact will be considered in assessing your statement.
On the question of trains leaving only 3 minutes apart, this represents a high frequency of service which is something passengers have been asking for - not complainnig about!
Sorry if its not what you want to hear.