The minimum age for part-time work in the UK is 13, with exceptions for modelling and acting. Age 13-14, the most common job is delivering newspapers, since at this age there are big restrictions on working hours. At 15, it's more common to work in a small supermarket, but I think it's become a lot less common than 10 years ago. (Of course, there are unofficial jobs, like gardening and cleaning cars.)
If this girl pays herself the whole £48,000 as a salary, the tax would be £12,693.20 leaving £35,306.80. Under 16s don't pay National Insurance (healthcare + unemployment etc tax), they would take home £39,600.00 from the same salary. The first £11,000 of salary is tax free, regardless of age.
https://listentotaxman.com/ will calculate this.
https://www.gov.uk/child-employment/minimum-ages-children-ca...