@criedariver
100% definitely no tip, even if they do a good job. Hairdressers, just like every other profession, are on minimum wage - and that is the lowest that they get. They're not being paid below minimum wage/a very low salary. So why are people still giving them tips? 
Dozens of professions don't get tips, like checkout operators, people who work in a fish and chip shop or takeaway, Travelodge Workers, Premier Inn workers, market workers, shelf Stackers, street sweepers, factory workers, McDonalds workers, refuse collectors, doctor/dental receptionists, pharmacy workers, dental nurses, etc, and also doctors, nurses, teachers, paramedics, police etc. You name it. We don't tip all them, do we?
This all harks back to a time when hairdressers - particularly for the first one or two years of their working life, were paid very little. I know that a friend of mine who had been working as a hairdresser for about a year, was only on £1.05 pence an hour when I was on £3.75 an hour - in an office ... (This was about 35 years ago.)
But now, since the Minimum Pay Act in the late 1990s, there is absolutely no need whatsoever to give anyone a tip. Hairdressers, along with Taxi drivers, and waiters/waitresses have always been given a tip. (Because their pay used to be very low.) But now there is no need for it at all.
I know people might say, 'but they really work hard.' But most people work hard. So why don't we all get tips?
Also, a £20 tip, for a £90 haircut/style?! Batshit. That's virtually 20%!
Also, her only doing half your hair, because she is double booked is grossly unprofessional, and is appalling customer service. This would have me never returning, and giving her a poor review online.