Literature Wales (formerly Academi) Wales based and national 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

Playwright's Calendar (list of comps specifically aimed at play/script writers)

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)