I don't actually know how the authorities could have made it work at Wembley.
On the presumption it would have had to remain a bubble (which would have raised some awkward questions if it hadn't have been) then how do you bus two lots of 40,000 fans down the M4 on the same day, and on a Bank Holiday weekend as well, the travel disruption it would have caused would have been unreal. And then when you get them there, where do all the coaches park, how do you keep the two sets of fans apart, and how the hell do you manage it afterwards?
They would have had to switch it to the Millennium Stadium imo. I can't see how they could have done it otherwise.