Skipness - the arguments concerning 'trickledown' relate to the true extent of its effects (in all instances, not just LHR). Hence my use of the word 'overwhelmingly' in the quote you highlight, as opposed to suggesting zero benefit beyond the SE. What is very clear, however, is that the regions will benefit by an order of magnitude more from long-overdue direct infrastructure investment than from vastly overpriced LHR R3 trickledown.
By the way, Skipness, the Liverpool - Manchester - Leeds (- Sheffield / Bradford) - York - Newcastle rail corridor is also a national asset in the same way LHR is. Although I do realise that you're probably choking on your lunch contemplating that concept.