National Youth Theatre The National Youth Theatre has training and performance opportunities for actors and technicians. Check out their website for news and advice about auditions and interviews (as a nationwide organisation the entry process is necessarily tough). They have a new book out called All About Theatre, packed with helpful info.
The National Youth Music Theatre So the focus is on music theatre with again opportunities not just for performers and musicians but also e.g. choreographers, designers and crew. Check out their website for news about auditions and courses.
Ideas Tap was a fantastic site showcasing lots of opportunities in the creative arts. It's closed now but we're keeping the link here because they've got great stuff in their archive section - including this blog post by Hon!
Hiive is a creative arts professional network which is worth checking out because a lot of the funding opportunities and competitions that used to be run on Ideas Tap can now be found here.