Beyond the Veil <http://www.gogpromotions.co.uk/> is a goth, darkwave 
and neofolk festival run by the people behind the Black Veil club.  It 
runs every Easter weekend, with the main bands event being on Easter 
Sunday.  Held in Leeds, it brings a wide range of international acts to 
the UK; past headliners have included ASP, Bella Morte and Diva 
Destruction.

Dark City <http://www.darkcity-edinburgh.com/> is another Easter event, 
and is based in Edinburgh.  Headlining acts have included Clan of Xymox, 
The Mission and VNV Nation.

Infest <http://www.infestuk.com/> describes itself as the UK's premier 
festival of alternative electronic music.  It is held every August at 
the University of Bradford Students' Union, and plays host to a variety 
of projects ranging from the futurepop of Assemblage 23 or VNV Nation to 
the harsh technoid rhythms associated with labels such as Ant-Zen and 
Hands Productions.

Carnival of Souls <http://www.carnivalofsouls.co.uk/> is a one day 
gothic/fetish festival.  It runs in Derby every October.

Flag Promotions <http://www.flagpromotions.com/> run a number of all-day 
events in London, including Gotham in May and Elektrofest usually around 
April or early May.

(Internationally there are a number of other large events of interest to 
UK goths, including M'era Luna <http://www.meraluna.de/> and Wave Gotik 
Treffen <http://www.wave-gotik-treffen.de/> in Germany, and Convergence 
<http://www.altgothic.com/> in the United States.)