What are the best newspapers / sites for open casting calls and auditions in the uk?

Sadly, there aren't any that I've heard of. Almost all websites are useless for getting auditions. There are very rarely any half-decent open auditions anywhere – most are just scams or publicity stunts - but when there are, thousands of hopefuls will be there fighting to be seen. And most of these will be experienced, qualified actors. Almost as soon as you get in the door (if you get in at all) they will ask what experience you have, what drama school you went to and ask to see your CV. So you need to have lots of impressive plays and roles on it. The best way to get genuine auditions is by signing with an agent. Almost all paid roles in anything are now solely cast through agents. It avoids directors wasting time with untrained and inexperienced amateurs turning up wanting to be auditioned. They don't have the time or money to do that nowadays, so by using agents they just get suitable-looking, professionally-trained and very experienced actors turning up. Sorry I can't offer any suggestions, other that getting an agent! Good luck anyway!