I'm 18 years old and have no degree (Taking a gap year), I work as a back end web developer at an e-commerce company. I earn £7.50/hr and have shown to the Owner of the Company and my manager how much I benefit the company, I have saved them £90/Week, and have made many systems which generate extra income, greater than the amount I earn.
My manager has recently delegated his work load onto me so he can do other tasks, so it shows that I have the competence to do his work, which exceeds my coworkers in skill, and they earn £13.75.
I recently asked my manager and the owner for a raise, this was negotiated over Christmas, I asked for £10/hour which I felt was reasonable, considering that the owner was very impressed with the System I made. When I asked my boss for the raise, we discussed the reasons why I should get it briefly, and he replied that since I don't have a degree, I shouldn't get as high pay, to which I replied that I am doing work equal to that of someone higher pay, so I should get paid more- he agreed and said he would think about it.
When he got back to me, he said that because I was late and absent 2 times in the last 5 months, as well as me creating a bug once (Which never went live), that I would have to show improvements and he will consider it again in 2 months, he then doubled my current workload.
I believe this is totally unreasonable, because I'm going to University in September I believe he is betting on the fact that I won't quit because I won't be able to find a higher paid job for such a short duration without a degree, to which he is right. If I left the company, my work would have to be done by my boss again, which would cost twice as much- they know I won't leave, so they are trying to extort me.
The reasons why I don't want to quit is because it's difficult to find jobs nearby that are equal to or higher pay, I live in a remote town- and I have no degree and I'm only 18.
All my co-workers agree that I need a pay rise, and I have only received compliments whilst working here, I know and my manager knows that I am an asset to the company, however because they know that I won't quit, they are getting the best deal they can.
I don't know how to assess this situation, I'm losing £750 even if I get the pay rise in 2 months since my time here is limited (~8 months left), I feel cheated and I don't know how to argue my case.