They normally issue you it at the roadside.
I refused to accept my one (as I wasnt doing what he said I was in the first place) and they posted me it, I posted it back to them and they posted it back again, I posted it back again with a formal complaint in writing, they investigated (ie they asked the copper who gave me it if he was sure, he said he was, it only needs to be in their opinion no proof is needed) I contacted the council to get video evidence to proove he was talking shit but sadly the camera on that roundabout was pointing the other way at the time. I said I would prefer to go to court then where I can have my say, and not accept the warning, and was informed that because I hadnt been summonsed I didnt have that option, there is literally nothing you can do!