Why ?
We don't even need a national stadium. Other countries seem to get by alright without one, and with the superb standard of stadia in this country nowadays, I don't see the point of building another one in the middle of England just to call it the national one.
The England football team...
They play 4 regular season games a year in England (currently at Wembley and Twickenham) and they always sell out, the demand is huge. I think a permanent NFL team based at Wembley would sell out no problem - there's only 8 regular home games a season anyway, so its not a giant leap from whats...