Perhaps "Target" wasn't a good choice of word, "Test Article" would have better described their intended use; you're correct in saying that the (large calibre) gunnery targets were just co-ordinates out to sea (where the expended rounds could be recovered from the mud flats at low tide for examination).
However, my understanding, from talking to people that worked there at the time, is that the airframes were subjected to live fire (smaller calibre) and other explosive devices to test their effect on modern materials and construction techniques in these "state of the art" test pieces!
It's just a pity that the various Ministers who decided the fate of these fine machines, couldn't be persuaded to sit in the cockpits during the trials.
