There are a lot of brow brands on the market now. I train in Mina Henna. I've been delighted with it and half my business has now switched over to henna.
Since I trained (Jan 2018) lots of new brands have been launched in the UK. None of these brands seem to be distributed by a big name company and most brands are terribly small - some don't even charge VAT.
I use Mina because it's made by an Indian company with years of experience, distributed in the UK by a small company with years of experience distributing Internationally sourced products. It's also the only International company legally compliant with EU regulations (obviously all the UK and EU companies are ok). I also know of at least one Big name company that would love to distribute Mina in the UK but Mina has signed an exclusive contract.
You can only buy Mina if you train in the brand and I think that's important. A brand that lets you buy their product without making sure that you can use it safely and well are frankly careless about safety and product reputation.
It's important to understand that it's not legal to offer skin staining services in the UK unless it's Mehindi henna art - which doesn't work for brows. Henna brows can only be marketed for its hair dye properties, not the lovely skin stain that you get for a couple of days after a treatment.
Have fun experimenting and bear in mind that henna application is surprisingly technical. If you don't understand enough about the product the results will be disappointing for your clients.