Do I need to get actors permission to screen my film?

Yes you do.
All on screen participants /actors in your film should sign a release form for you allowing unlimited use of their participation in your film for all time and in all circumstances and formats, this is industry standard and good practice if you ever intend to publicly screen your film, you must carefully keep the originals of each release signed and dated, you never know, beyond success at Cornwall Film Festival there are other festivals and there is even are Oscars for shorts so don’t cause yourself future problems by missing this vital step at the start!

Check out the useful BBC website: http://www.bbc.co.uk/dna/filmnetwork/clearances

This answers most copyright questions