you know what they say about plans, they never survive first contact with the enemy.
I dont believe for one minute hitlers troops could have been able to invade, we had a very good coastal defence and still had a strong navy, the only choice would have been from the air initialy, paratroopers, lightly armed yes but they only need capture and airfield or two, with only air superiority as apposed to air supremecy they would have suffered heavy losses, but once, or rather if they had established a bridgehead, they could have got enough equipment across to employ blitzkrieg tatics and we know how effective that was.
I think we owe more to that fact we manged to hold them off during the battle of britain than we realise.