[ClumsyPilot]

> which are commercially produced, tested and sold at retail/wholesale?

That doesn't mean they won't kill you -> try drinking bleach or pesticides for demostic plants, both avaliable at the nearest supermarket.

Barber chemicals can give you skin burns or render you partially bilnd. That's mostly female products though, being a men's barber is typically simpler.

that being said, most of UK has no special lisence for barbers -> just need to follow health and safety and have insurance.

https://www.gov.uk/register-a-hairdressing-business