All hosts should have insurance. Most take out Public liability, but it is usually Employers' Liability which is needed. We encourage hosts to talk to their current providers about cover.
WWOOFers - you are responsible for your own safety. Check you have insurance that covers medical care including transportation home, accidents, and loss of property. You should also be covered against you injuring other people or losing or breaking other people's property. Make sure your insurer knows that you will be a volunteer on a farm.