Labelling someone a 'consultant' won't work, if in fact they are behaving and being treated like an employee.
If the tax authorities or an employment tribunal decide that the 'consultant' is really your employee, they will ignore the label and treat the consultant like an employee and you may lose any advantages. This can include having to pay tax payable under a normal employment relationship.
To avoid this situation, you need to be very careful about what's in the consultancy agreement, particularly around issues such as how much work you have to provide, how much you control/supervise the provision of the services, the length of the appointment, whether the consultant is integrated into your workforce and if the consultant can work for anyone else. Consider using Rocket Lawyer's Consultancy agreement to help ensure you keep to the rules.
You also need to be particularly careful about confidentiality and intellectual property created by the consultant. Once again, consider using Rocket Lawyer's consultancy agreement to help ensure you stay legally safe.