D
I assume you mean that you are trying to cancel the hearing and not the appeal.
To be practical, your health is more important.
Can you chase the Cardiologist in the morning? I'm just conscious of the delays that can occur in the NHS.
The bottom line is that the Judge can make a decision on the day as to whether to hold the hearing or not. They have a copy of your letter explaining about the surgery, hopefully this will mean that they understand the importance of the appointment. If they go ahead and the Decision is not what you hoped for then you can ask for a Set Aside of the Decision, based on the fact that you were unable to attend.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems