You could potentially use (clearly not 'employ' as that's a contradiction) a freelancer who invoices you for services only if that person regularly goes around offering such a service to many shops and businesses. In other words, he or she is a self employed shop manager who moves from one to another regularly somewhat like a locum doctor or dentist. In rpactice it's not going to happen.
Also, further to earlier answers, NI, both Employee's and Employer's, becomes payable on payments of over �105 per week. It was �89 for the tax year 2003-04 but has gone up since.