I used to work in a salon doing hair & beauty, we had to pay 25 to get our colour done but she never charged us for getting hair done for weddings or anything, but we usually had to work for a few hours in the morning and then once the wedding rush was over we could finish and get ready ourselves! I always found it a bit awkward getting anything done because I wouldn't be sure if I had to pay or not, there was no official policy so most of us never really got anything done!
If one of the hairdressers was going to have to come in early for you I would rather just go to her house or something and give her a few pound as thanks!
Amanda xx