The gala will take place on October 6, 2015 in London. The Circle's filming is supposed to stop that same month.
CelebIntel says it's confirmed:
Judi Dench, Emma Watson, Colin Firth, Meryl Streep & Tom Hiddleston are confirmed for @BFI London Film Fest events - http://t.co/cFlulkFVgD— CelebIntelUK (@CelebIntelUK) September 29, 2015
I think their info might come from this:
The BFI can also today reveal the names of leading lights from around the world who have come out in support of the LUMINOUS Gala and Film Is Fragile campaign, with names including Tom Hiddleston, Dame Judi Dench, Martin Scorsese, Emma Watson, Dame Maggie Smith, Colin and Livia Firth, David Oyelowo, John Hurt, Paul Greengrass and many others named LUMINOUS Gala Champions and proudly supporting the work of the BFI.
Emma will be one of the LUMINOUS Gala Champions 2015. John Boyega, The Circle co-star, is also one of them.