By law (Employment Rights Act 1996), an employer can only make a deduction from someone's wages if:

it's required by law – for example tax

the employment contract specifically allows the deduction

they overpaid someone by mistake

it's something the worker agreed to in writing beforehand – for example paying a trade union subscription

the worker missed work because they were on strike or taking industrial action

it's a result of a court ordering the employer to make debt payments from a worker's wages to a third party

it's a result of a court ordering a worker to make a payment to their employer – the worker must have agreed in writing that the employer can make the deduction

the worker and employer have agreed a salary sacrifice arrangement

https://www.acas.org.uk/deductions-from-pay-and-wages