Below are some competitions that look good! Note, some of them are annual, so don't worry if you've missed the deadline, it will come round again! And please note that, although I check this page as often as a busy author can, some of these competitions might not exist any more!
The ALCS Carnegie Shadowing Scheme Writing Competition
National Literacy Trust for KS1 and Lower KS2
National Literacy Trust for Upper KS2 and KS3
The Write Stuff for girls aged 11-18
Project Remix for 14-19 year olds
Radio 2 500 Word Writing Competition
The Henrietta Branford Writing Competition
The Hot Key Young Writers Book Prize
The Red House Young Writers Competition
Wicked Young Writers Competition
Random House Anti-Bullying alliance
Chorleywood Bookshop Young Writers Competition
This is one to get your school involved in, if you are in year 7 or 8