When mine expired I had to send it (green paper one, never any points entered, as issued some 40 years ago) as part of the application. I asked to get it back with other documentation but was told they won't return it because "it is worthless". To me it is of great sentimental value and, yes, it is of no value to anyone else - the woman was quite uppity/officious about it and it has vanished into the general mess that is the DVLA. If you want to keep yours as a memento/souvenir then the only way to do so is to lose it.