The Employment Rights Act allows an employer to recover overpayment of wages, which includes expenses and other allowances. Such a deduction from wages would therefore be lawful.
However, where a worker genuinely believes he or she was entitled to the overpayment and has already spent the money (usually on exceptional or non-regular purchases) in good faith, a civil court might sometimes decide that the employer was not entitled to recover it.
If you think you have been overpaid, you should contact your employer immediately to check whether or not you are entitled to the ‘extra’ money. Not only should this stop you spending money to which you are not entitled, it could make it harder for your employer to later deduct the amount in question from your wages on the basis of an overpayment if they failed to act after you alerted them.