Yes you will be lucky to earning much more then £2500 a month for the first 10 years. No overtime pay, no extra pay to work in nights or holidays, no summer holidays, no holidays at all except when you are in low peak season.
Your high school grades are not very important, the maths is not very complicated, it's not really difficult the theory part, just time consuming.
Overtime does not exist, take you pay, and you will work as much as they need you for this month. 150 - 250 hours a month.