There's a National Minimum Wage, to specify the minimum amount that your employer has to pay you, but this varies according to your age, so older workers could well end up being paid more than you if your company is paying everyone the minimum.
Your mate is being paid the legal minimum for 18 year olds as the minimum wage for 18 - 21 year olds is currently £4.83 per hour (It's £5.80 for those aged 22 and above). The minimum wage rate for 16 and 17 year olds starts lower at £3.57 per hour.
