My take is that;
1) The Germans came up a well organised air defence system - unknown elsewhere at that time
2) The UK had very good short range combat aircraft although limited in numbers but fighting over home land
3) The change of German tatics from Sept 7th was crucial to the outcome.
4) The use of resouces by Park was crucial in so much that nearly all the incomming raids were met with some resistance which undermined moral as the Luftwaffe crews were constantly being given the impression that the RAF were a beaten force.
The German High Command seemed more concerened about weather conditions in the Channel for September than the British Navy especially as they didnt really have the best type of troop transport for a cross Channel expedition.
I think that if the surface ships of the Navy had sailed down the North Sea towards the Channel without RAF air cover the Luftwaffe and U boats may well of actually 'slaughtered' them. I seem to recall that certainly in the early part of the war the RN ships didnt do to well when up against a concentrated air attack.
It would of been imperative for the Germans to capture and hold a port and airfields to make the invasion work and then keep the troops supplied. I dont think that in 1940 they had that capability - they would of relied heavily on a British collapse similar to the French.