The Employment Rights Act 1996 permits an employer to make deductions from a person's pay packet if such deductions are:
(1) authorised by legislation (such as income tax and national insurance deductions) or
(2) authorised in your employment contract or
(3) you have otherwise agreed to them in writing beforehand.
Therefore, in general, if you have not agreed to a deduction in writing, the employer cannot dock your pay!
However, there are special provisions relating to retail workers.
An employer may make a deduction from wages or demand payment from a worker on account of one or more cash shortages or stock deficiencies.
"Cash shortage" is defined as "a deficit arising in relation to amounts received in connection with retail transactions".
"Stock deficiency" is defined as "a stock deficiency arising in the course of retail transactions".
Therefore, if your till is short and you work as a shop assistant, your employer may make deductions from your wages.
That said, it is unlawful for an employer to deduct more than 10% from the gross wages of a retail worker on any one pay day.
Accordingly, where deductions can be made from a retail worker's wages to pay for shortages or stock deficiencies, the sums owed may be recovered in instalments but each instalment must be no more than 10% of the worker's gross wages on any one pay day.
However, the 10% limit does not apply to deductions from the final payment of wages.
With certain exceptions, a deduction of any size from the wages of a retail worker is unlawful if made more than 12 months after the cash shortage or stock deficiency to which it relates was (or ought reasonably to have been) established by the employer.