It depends on how long the sentence is. Also to some extent I suppose what the crime was and whether it could affect your job. See this:
Question:One of our employees recently came up in court, and was sent to prison. Surely we can dismiss him?
Be careful. Even though your employee has been convicted of an illegal act, you should still carry out such investigations as are reasonable in the circumstances before making any decision. Such investigations must be carried out promptly to ensure that any dismissal is fair.
If the employee's offence was related to his or her work, a dismissal would probably be fair on grounds of conduct. Alternatively, the circumstances of the imprisonment might be such as to reflect adversely on the image of your business, and/or destroy trust and confidence in the employee, which might count as 'some other substantial reason' for dismissal. But great care should be exercised and legal advice sought.
The fact that the employee is in prison and therefore cannot do his job is not in itself enough: it may 'frustrate' the contract, but as a general rule Employment Tribunals are reluctant to decide that the contract of employment has ended because of the employee's inability to perform it