The date an employee actually reads a letter dismissing them will be the date their employment ends (provided they have not unreasonably ignored it). The Supreme Court has just confirmed this beyond any doubt.
This means that the termination date can be considerably extended, even if someone has signed for a recorded delivery.
The exact date of dismissal can be critical for all sorts of reasons. For instance, when deciding whether the employee has been employed for a year so they can bring an unfair dismissal claim.
So, make sure you dismiss employees face to face, and hand them a letter of confirmation there and then.