Academi (Welsh literature website but has nationwide comps)

Literature Training (List of Markets, comps and other writing opportunities)

The National Association of Writers Groups 'More Competitions' 

Poetry Society's list of poetry competitions

Prize Magic (informative and quirky)

Ralan.com (competitions specific to the humour, dark fiction and speculative genres)

Scribble Short Story Magazine Quarterly Short Story Competition (£3 per entry - ongoing comps no closing date)

The Short Story list of short story competitions

Writers Forum (monthly competition)

Writers News/Magazine (subscription required for WN comps)