I'm sorry to sound rather negative but you're going to find it hard getting employment. The law states that you can't work more than 2 hours per day (on schooldays) and you can't work after 7.00p.m. Putting this together with the fact that you can't work on Thursdays means that you're only available for a maximum of 8 hours per week on weekdays and generally at times when most hairdressing salons don't have many customers.
Your best chance of employment would be on a Saturday but, at 14, you're legally restricted to working a maximum of 5 hours on a Saturday. (This goes up to 8 hours when you're 15). So, until your next birthday, it might be hard to get Saturday work.
As I've said, you're going to find it hard to get employment but it might be worth phoning all of the hairdressing salons in Harlow to ask if they need any Saturday help. (Remember to state that you're already studying hairdressing). Even if they've not got any vacancies at the moment they might keep your contact details in case something becomes available.
Chris