It hasn't been "cancelled" it was simply never scheduled to air as a result of something else ITV wanted to broadcast, in this case a friendly football match, which was announced in July and therefore a fixture in the schedule, long before IAC broadcast run was set in stone and something they've known they had to plan around.
Besides it's rather more than just the usual pointless friendly, it's been a long time since England has played in Scotland and this will be played in the spirit of a full blooded competitive fixture, there won't be anything friendly about it. As a result there's a lot of interest in this game, far more so than the qualifiers in fact. Sky sports are even showing the game at the same time.