It's possible that 10 years for the same firm might be regarded by an Employment Tribunal as amounting to employment, in which case you'll have all the rights of an employee. But it's not cut and dried and it's got nothing to do with HM Revenue & Customs. IR35 is completely irrelevant unless you've been paid through an intermediary, usually a limited company of your own. And even if the IR35 rules did apply (and they only came into effect in 2000) that still wouldn't mean you were an employee.