Mike is quite correct you are probably thinking of is a restraint of trade condition, you do have a protectable interest and therefore could have inserted as an express condition, anti-solicitation or a restrictive covenant to try to avoid poaching within a contract of employment, even then it would have been for a limited time scale and limited area and may have been challenged on the grounds that the employee was unable to engage in their trade without undue restrictions, you would have obtained this employee under the TUPE regulations 1981 and it may have been difficult to then impose these express conditions. Without a written contract of employment, including these restrictions you have little legal remedy.
You could write to your ex-employee and say you believe it to be morally wrong (a popular phrase nowadays) to use your client base to start her business, but if she chooses to ignores you will just have to compete, you will be aware that this is quite a common occurrence in your industry.