Crowds in London for Test Matches:
2005: Great Britain v New Zealand - Loftus Road
Crowd: 15,568
2004: Australia v New Zealand - Loftus Road
Crowd: 16,750
2000: Australia v England - Twickenham Stadium (World Cup Opener)
Crowd: 33,758
1997: Australia (SL) v Great Britain (SL) - Wembley Stadium
Crowd: 41,135
1995: Australia v England - Wembley Stadium (World Cup Final)
Crowd: 66,540
1995: Australia v England - Wembley Stadium (World Cup Opener)
Crowd: 41,271
1994: Australia v Great Britain - Wembley Stadium
Crowd: 57,034
1992: Australia v Great Britain - Wembley Stadium (World Cup Final)
Crowd: 73,631
Total: 345,687
Average: 43,211
Crowds in Northern England (Against Australia only)
2005: Great Britain v Australia - Hull
Crowd: 25,120
2005: Great Britain v Australia - Wigan
Crowd: 25,004
2004: Great Britain v Australia - Leeds
Crowd: 39,200
2004: Great Britain v Australia - Wigan
Crowd: 25,011
2004: Great Britain v Australia - Manchester
Crowd: 38,572
2003: Great Britain v Australia - Huddersfield
Crowd: 24,163
2003: Great Britain v Australia - Hull
Crowd: 25,147
2003: Great Britain v Australia - Wigan
Crowd: 24,614
Total: 226,831
Average: 28,354
So looking at it from a purely numbers sense, the South certainly holds its own against the North. Sure there are some factors that skew it in the South's favours, such as the quality of some of the events, and the larger capacity, but its fair to say people from the North enjoy a trip down to the capital, and there is a supporter base in the Southern half of England that can hold its own.
The fact that half of the viewers who watch Super League are based in the Southern half of the UK means there is a huge number of people who watch the game but don't attend matches. An event such as Australia v England at Wembley Stadium is something that could get them out.
As for the argument about France not being able to compete in the four nations, here is the French's teams recent results against the big four:
2004: France 20 - 24 New Zealand
2004: France 30 - 52 Australia
2005: France 12 - 44 Australia
2007: France 14 - 42 Great Britain
2007: France 16 - 21 New Zealand
2008: France 8 - 56 England
Some touch ups, some close results, fair to say they are a side victim of only playing one off clashes. Would they win a game if they joined a Quad-Nations tournament? Possibly not in their first few years, but they would offer a lot more than say Italy in the Rugby Union 6 Nations.
I think with Toulouse joining National League One in 2009, and the Les Catalans only to get better, the missing link in the redevelopment of France is having them included in a high quality international tournament like the Quad Nations.