'After GWC descend FL120' wouldn't work because then you might level off at FL130 if that comes up before GWC, and then you'd be moaning for further descent.
All I have to add is to say that while technically a clearance to a lower level than the one originally mentioned in the restriction cancels the restriction, you would be have to be pretty low down in the common sense queue to assume you can blindly ignore a restriction. IF IN DOUBT - ASK.
It is apparent that most UK ATCOs who contribute here seem to agree that they want to reduce not increase RT loading by assuming that you will comply with previous restrictions unless they cancel them. For example, an initial clearance through COA might be 'Descend when ready FL 300, expect FL 250 by or abeam LOGAN'. The onward clearance to FL250 might come in several stages, and I would argue that you have been given the restriction in the 'Expect' and unless that is cancelled you should comply with it. To restate it every time would massively increase an already busy R/T.
Might it not be a good idea when flying in UK airspace, therefore, to keep this in mind??