Goodness, I hope you dont get that worked up in the office
There is a famous saying - ask 2 doctors, get three diagnoses. So you have an abnormal ECG but disagreement as to the cause. It may even be that the CAA are relying on the computer print out - search this board for loads of cases about that. Sorry, frustrating it may be but there are no grounds for legal action. The CAA is not forcing you. It is your choice whether you want a medical certificate or not.
I would suggest you take the letters from your three cardiologists and send them to the CAA asking them to review the ECG. Include the more recent ECGs you have presumably had in case the initial ECG was faulty. The CAA will then take the opinion of their consultant cardiologist. You may be pleased with the outcome.