This is not a simple issue with a yes or no answer. The legality of recording conversations at work is a grey area, evolving all the time. The Employment Appeals Tribunal has allowed it in some cases but dismissed it in others. It depends on the circumstances of each case and how relevant the covertly recorded evidence might be to a case. You should talk to an employment law specialist about secret recording, if you can.
Can an employee secretly make recordings at work? Tread carefully. Employers are wising up to the issue and training managers and HR staff to expect that employees might try to secretly record conversations in the workplace.