If you are an hourly-paid worker, you should be paid the agreed contractual rate for every hour you are required to work. 'Work' includes time spent at the employer's premises and at the employer's disposal, even when you haven’t started on your tasks yet because you are getting ready for your working day.
If your employer expects you to be on the premises everyday 15 minutes early, this is likely to be working time for which you should be paid. Not paying you may be an unlawful deduction of wages.
If failing to pay you for the extra 15 minutes means that your average basic hourly pay during your ‘pay reference period’ falls below the National Minimum Wage, your employer may also breach minimum wage laws. Your pay reference period is based on the pay arrangements where you work. It will be a day if you are paid daily, a week if you are paid weekly and a month if you are paid monthly.