It makes no difference whether the client self administers or not:
"You may have been told that offering tooth whitening treatment to individuals by handing them trays for self-administration and asking them to sign consent forms circumvents the law:
False.The Dentists Act makes it illegal for anyone who is not a dentist to give “
treatment, advice or attendance” that would usually be given by a dentist. Handing an individual a tooth whitening tray and advising them on application, amongst other things, could constitute the giving of “
advice or attendance” and would be illegal.
The GDC has successfully prosecuted several individuals for illegally practising or offer to practise tooth whitening treatment using the so-called ‘self-administered’ method of providing the treatment."
Source:
http://www.gdc-uk.org/membersofpublic/illegalpractice/pages/faqs.aspx