I found that quite a lot of the film festivals on the British Film Institute were either out of operation or not yet open for this year. That being said their little table was very handy for finding a lot of links.
From The Book : In Short: A Guide to Short Film-making in the Digital Age
BFI. (2016). Film festivals for young filmmakers. Available: http://www.bfi.org.uk/education-research/bfi-film-academy-scheme/film-festivals-young-filmmakers. Last accessed 21st Nov 2016.
Elsey, E. and Kelly, A.. (2002). Selection of festivals showing short films. In: Elsey, E. and Kelly, A. In Short: A Guide to Short Film-making in the Digital Age. London: British Film Institute. p152-155.
Raindance. (2013). World’s Top Short Film Festivals. Available: http://www.raindance.org/worlds-top-short-film-festivals/. Last accessed 22nd Nov 2016.