Keep an eye out on the artist wanted section of Creatabot https://creatabot.co.uk/category/creative-artist-wanted/

Without a shadow of a doubt the jobs section on the arts council website should be your first point of call when looking for jobs and artist commissions. http://www.artsjobs.org.uk/

Another place to look is on the Guardian website –  http://jobs.guardian.co.uk/jobs/arts-and-heritage/

The Stage Newspaper and Online is not just for actors and actresses –  http://www.thestage.co.uk/recruitment/

Creative Boom have a jobs section – http://www.creativeboom.co.uk/jobs/

A smaller site – http://www.periscopeuk.com/

For film and production – http://www.filmcrewpro.com/uk/jobs.php

This is an open article – please add where you also find is useful in the comments box below.