I, like many instructors am self-employed. There is no way I could be considered an employee. I choose where and when I accept work. I provide my own tools of the trade ( charts, approach plates, etc ). I pay for all my medicals, revalidation flight tests etc. I am vat registered and charge vat on all my invoices. I also decide how much I charge per hour, or day, and what expenses I pass on as a charge.
Anyway, its how you work that determines whether or not you are self employed or an employee. It is not determined by the type of work you do; which in this case is instructing