You might need to think about this from a different angle.
For instance, it's not the same as being trained in Acrylics at college and then choosing to use a different brand of Acrylic on your clients. That's perfectly fine.
However, if you trained in gel and advertised a 'Shellac manicure' but used say, Gellux on your clients, you'd be guilty of Misrepresentation.
With HD Brows, its ALL about the BRANDING. They have spent £'s to protect their brand.
If you advertise to clients that you offer HD Brows, you are legally obliged to be trained by HD Brows and to use their products and follow their steps exactly as stated.
Alternatively, you can choose to use a variety of products and advertise a 'brow treatment service' but you can't legally call it HD Brows, otherwise, you risk being sued.
Does this help explain the situation?