Extra work - Voice Over

Advice please --- I have been contacted for the possibility of doing a "voice-over" recording in English (I have done one for my full time employer for an English language PR video - apparently I have clear accentless English!) -- In the unlikly event that I am chosen by this external organisation. How do I go about staying legal and getting paid? (only going to be a couple of hours if I am chosen) Do I need to be TVA registered? - How to do that? Thank you for your time