Because I was clueless about this, somehow, and I have been merrily posting my £1 coins into DD's terramundi pot (you know, the money boxes that you have to smash when they are full). By the time it's full enough to smash, the bloody things will have been out of circulation for about 5 years!
I thought DP was winding me up, but sadly the internet is well and truly with him on this one, so I just thought I'd flag it here in case anyone else has been doing something similar.