An employee is not treated as dismissed for statutory purposes until they know of the dismissal. It is therefore best practice to communicate a dismissal decision face to face, either by handing over a letter or orally and then confirming the decision in writing.
Where a letter or email is used to communicate dismissal, the termination date for statutory purposes will be that on which the employee reads the letter or email (unless the employee has deliberately delayed doing so). The dismissal date could therefore be delayed if a letter is posted and the employee is away on holiday or sick when it is delivered. In some cases this delay could allow an employee to accrue a year's service and become eligible to claim unfair dismissal. (Gisda Cyf v Barratt, CA)