So the argument for not turning LHR into a major hub airport is that it isn't one already? Well there is common sense thinking for you.
Yes people can transfer via AMS. Heathrow wants to bring that traffic and income via the UK which will bring money into the economy.
This £18bn figure is the very top estimate and it is more likely to be much much less than that.
As for an expanded LHR being of benefit to the whole of the uk? Of course it is. In case you haven't noticed the people of the south east already have access to Heathrow. This is about giving access to the rest of the UK and Europe. Moving short haul to another runway will free up the longer runways for more long haul routes too.
I was waiting for shed to chime in with his north/south chip on his shoulder but the reality is an expanded LHR benefits all the UK as much as it already does the south.