Just typical I find out about this after the event has just finished... did any one go?
The Great Seed Swap is being held at Kew's Millennium Seed Bank on 20 October 2012 at Wakehurst, which holds the largest wild seed collection in the world, and on the surrounding Wakehurst estate.
The aim of the event is to encourage and inspire more people to grow vegetables and plants, and experiment with different varieties – particularly heritage vegetables that were once the mainstay of British gardens.
Gardeners will be able to swap seeds from plants they have grown at home for seeds from a range of garden vegetables and flowers, grown by other enthusiasts.
http://www.kew.org/visit-wakehurst/whats-on/seed-swap-20oct2012.htm
I didn't go, bit far for me, but looks really interesting. Seems they have moved the page in your link but I had a google and found these
http://www.kew.org/news/the-millennium-seed-bank-joins-seed-swap-events.htm
http://www.bbc.co.uk/news/uk-england-sussex-20015045
Hopefully they will be repeating it next year.