Henri, you have to understand one thing about paratroopers, though:
First of all they have no heavy equipment. No artillery, no Stugs, nada. That's not ideal in any case, but even less so if you want to do some city fighting.
Second, you lack supplies from that point on. Paratroopers are useful for a surprise drop and grab some undefended objective, but you have to move in some real army there FAST. Because paratroopers lack the "endurance" to hold anything for more than a day or two.
Third, at the time they tended to come down with just a pistol on them. Anything else, including rifles, came down in separate paradropped crates. So they'd then have to gather themselves, find the crates, and arm themselves.
And that's one reason why you don't want to paradrop in a city. You really don't want to be in the park with your pistol, your other equipment to be in a whole other park or on a house, and only God knows where your officer landed. You're very vulnerable in that time.
Fourth, you're even more vulnerable in the air while you glide down slowly, and your airplane before that is a weak transport airplane too. You REALLY don't want to paradrop over a place with massive numbers of FLAK batteries. Such as, you know, London.
So while I'm not surprised that the British generals thought of the possibility too -- it is their JOB to think up every scenario in advance -- I wouldn't take it for all that huge a threat.