The short answer is:- “When they are working for themselves, being responsible for the content and delivery of their own programmes”
Looking at it from an employer’s perspective, is the freelance instructor covered by your employer’s liability insurance? (therefore not needing personal PL insurance). HSE guidance says: Whether or not you need employer’s liability insurance for someone who works for you depends on the terms of your contract with them. This contract can be spoken, written or implied. It does not matter whether you usually call someone an employee or self employed or what their tax status is. Whether you choose to call your contract a contract of employment or a contract for services is largely irrelevant. What matters is the real nature of your relationship with the people who work for you and the nature and degree of control that you have over the work they do.