Don't admit or make it seem like you're admitting guilt as most officers carry body cams which can and will be used in court. When you receive this letter it will also state how to appeal, I'd recommend lawyering up and telling your police force you are, you'll find that the charge will be dropped in most cases. Police forces are stretched thin in terms of resources and it is unlikely their going to spend resources over a fine and 3 points. As long as there is no photographic or video evidence of you running the red, and you did not admit or imply you did when you spoke to the officer, then it is really his word against yours. Most likely was a slow night for the officer and wanted to ruin someone else's night.