Unfortunately they can do this in the retail trade if it's due to '
shortfall from a cash or stock shortage'. If you've got individual sign-on codes to the tills then it would seem reasonable that the employer deducts money only from those who used the till. You could then try to argue that it was an unlawful deduction from your wages.
There is a restriction that means that '
the employer is not allowed to take more than 10 per cent of your gross wages from your take-home pay on any individual payday'.
See the ACAS website
Protection from unauthorised deductions.
Or contact the ACAS helpline 08457 47 47 47.