I went on one of the St Johns Ambulance 3 day First Aid courses when we moved into the sticks. I am not sure how much I would remember now, but it would still be more than I knew before I went on the course, and I would be more confident also.
As has already been mentioned, the defib's are VERY easy to use and could easily make the difference between life and death, so I salute what you are doing 100%.
A couple of other very basic things which I took away from my course:
1. The emergency (999) call centres do far more than just arrange an ambulance to attend. They also talk you through emergency basic 1st aid, primary checks, and how to do CPR if required, so you are not 'on your own' so to speak. It sounds obvious, but that support at the other end of the telephone had never registered on my radar at all.
2. Do not underestimate how hard it is to carry out 10-15 minutes of CPR on someone while the ambulance arrives. It is absolutely exhausting. Adrenaline may get you through, but if there is someone else nearby to take turns that is a massive help. Get them to watch what you are doing so they can take over from you every minute of two. Getting your technique right will help to save effort.