The implication is you are providing refresher training for people who already hold ATC licences but you have no ATC licence yourself.
ATC Instructors normally have ATC Licences to carry out training for 'approved courses' and for your course to have any meaning in the eyes of the CAA, you would need their authorisation to conduct an 'approved course'. To become approved would be prohibitively expensive due to the need for simulation facilties etc.
I would suggest you direct your clients to contact the training section at your nearest ATC unit.