If the individual is truly self-employed you and she would be entering into a commercial contract. Providing both parties agree, it can contain whatever legal terms you wish, within reason. Consult a solicitor specialising in this field, as to go ahead without a written, comprehensive and signed contract would be a recipe for disaster.
Employment Tribunals will not just look at the paperwork, they will also consider the actual relationship at the time they consider the claim from the claimant. Relationships do change over time, and whilst the Tribunal might well accept that at the start it was a true contractor/sub-contractor relationship, some years later that might not be the case.
You need advice, therefore, on what constitutes a commercial contract; a worker’s contract under which a worker contracts to perform personally some task or duty and that of an employee. You would then need to monitor the relationship on an ongoing basis and take steps as appropriate to reinforce the desired one. It may in fact be simpler to employ her in the first place and keep all the profits.