You`ll need the help of a Gestor in setting yourself as Autonomo, they`ll deal with all the legal stuff and advice.
Gestor doesn`t really excist in the UK, there similar to an acountant come lawyer.
Most cost around 25.00 Euro`s amonth, they sort out your tax etc etc.
Then dependant on your area, depends on how well you do.
If your Costa Del Sol/Costa Blanca area, I`d look at another type of job, there are hundreds of Brits doin it.
There are even brit beauty schools training to NVQ`s in Fuengirola.
Marbella is full of mobile and static, doin everything you can imagine.
Give you some idea I googled mobile hairdressers in Fuengirola, one directory sites listed 38 businesses.
Give you an idea here
http://costadelsol.angloinfo.com/af/564/costa-del-sol-hair-and-beauty-mijas-costa.html
Its similar to to Construction and general building work, the markets full of Expats from those trades.
Location is very important, you need to find an area with enough Brits to survive on, but not enough to attract competition.