I have a post running just now - 'Alternatives to Trafalgar Square'. I have decided to change my initial opinion but don't know how to edit that original post?? In my initial post I suggest Russell Square along with a few other options. Having listened to lots of opinions and done a bit more research I think Granary Square would be better suited. It is in the same proximity as Russell Square so still easy access for the Tartan Army arriving into Kings X and Euston Stations, still easy direct tube line to Wembley from Euston Square, it has pubs, it has no grass (worry about potential mudbath) and it even has fountains!!
The other option is Covent Garden which looks really nice but not so well suited to a mass gathering but great if you want to spend the day in the pub or on a terrace!