Abnormality is not really the best word to describe such issues on an ECG, that would be normal for you and probably 30% of the population. The software in the machine is not that smart.
The only written disclosure that you are asked to make is if you have had ever had a medical denied, at this stage that is not the case. With all probability you will get the medical, just a protracted length of time. Try and clear it up before the interview.
I would not mention it unless specifically asked. If you are asked, have the CAA clearance with you in your bag.