Mad jock has good advice, and you might even find that the "self-emplyed" pilots were not to be considered self-employed. The definition is complicated, and flight instructors are on a bit of a concession anyway, but expecting you to be on site without you being able to invoice for it might swing things. However wait until you are about to leave voluntarily if you want to rely on that, they might not be too happy - they are liable for isues regarding self employment / PAYE status, not the (self) employee. They could be in some trouble with the tax man, as well as having to pay you minimum wage, backdated ...