Hi Rob,
The best thing you can do is call the CAA Aeromedical Department and ask to speak to a Cardiologist, or obtain a number at which you can call one. This person, who specialises in the study and treatment of heart conditions, will be able to tell you exactly what the problem is. You need to find out exactly how serious your problem is and if they feel that it would pose a threat to aircraft operations and why. When armed with all the medical and technical information you can get, you could try calling the British Airways Medical Department and try to speak to the Chief Medical Officer. With an anonymous conversation (remember that doctors may not divulge personal information disclosed to them) he would be able to tell you whether or not you would make it in to someone like BA- although do bare in mind that BA requires slightly higher than usual medical standards.
Please remember though that the CAA would be unlikely to issue you the medical if they thought it was going to be a big problem for you!
E-mail me if I can be of any more help!
FD