There's no statutory (ie legislative) provision for that and it's governed by common law (ie case law) principles.
It's settled law that an employer's counter notice will be treated as a dismissal in these circumstances and will serve to bring the termination date forward (this case deals with that fact - http://www.bailii.org/uk/cases/UKEAT/2012/0455_11_0902.html)
But again, at common law, the employees compensation for unfair dismissal will be limited to the date on which their original notice would expire.
Hope that helps.