A universal definition of myocardial infarction is offered at http://eurheartj.oxfordjournals.org/content/28/20/2525.full of which the most important words are “accompanied by”.
Myocardial infarction is a definite diagnosis, and one which can be proved or disproved, but only by technical means which are unlikely to be available to a GP at a sudden death, or arguably by history and observation over a period – which is by definition unavailable in a sudden unobserved death.
Ischaemic heart disease is a cause of death in which one may reasonably believe, based on prior history and manner of death and which is generally assumed to have had a long course, therefore a doctor acquainted with the patient is highly likely to have attended or treated them in their last illness.
The cause of sudden death in IHD is empirically not usually myocardial infarction. It seems likely that either a sudden demand on the heart outstrips the limited coronary blood supply leading to a reduction in output and pressure, and a rapid and quiet feedback spiral giving no cardiac output and prompt death – but not an infarction; or that an area of myocardium rendered ischaemic gives rise to an unrecoverable dysrhythmia with prompt collapse and death. But not infarction.
Cremation certificates require the signatories to be without doubt as to the nature and manner of death. This is beyond “balance of probabilities” but is not a demand for an exact and precise diagnosis.
Being persuaded to give a definite diagnosis of myocardial infarction on a death certificate and cremation form when you not merely cannot be without doubt about it, but are provably unable to have such certainty is unwise, not least because the rejection of the cremation form causes surprise and alarm among the family who may well be your patients.
A repeated statement “I am not in a position to certify a cause of death” may seem unhelpful, but if true is not unreasonable. If the cause is “obvious” to someone else, then let them certify it.
In the case of a patient known to you to have had (a diagnosis of) ischaemic heart disease, and who dropped dead or died in the night without fuss, if you form a clear opinion that the cause of death was ischaemic heart disease and certify this on the relevant forms it seems extremely unlikely that any trouble, disturbance or adverse comment will result. If you have occasion to discuss it with HM Coroner and agreement is reached it seems literally incredible that repercussions would ensue. If of course you do not form such an opinion, see paragraph above.
 Doctors in Devon have been persuaded to give MI as the cause of death. A post-mortem has never in my experience shown an MI, and contrarily has excluded an MI in each case. Ischaemic heart disease has been present in all save one. A view that more post-mortem examination should be done than currently in order to improve medical knowledge and be certain of more cause in more cases is a perfectly reasonable view, and merits discussion, however the post-mortems should done at the expense of the State, not the estate of the deceased, and at the request of the attending clinicians not that of the Crematorium Referee.