If there was a doctor on board he would have advised to divert the plane, believe me - or he made a wrong diagnosis, I have to look that case up. Especially with light of a very long flight I would have asked for an immediate diversion. Even if the patient is fine during a long haul flight complications can come later in the flight and than at a time when you cannot land any more - then what. And, as already has ben pointed out, you want to get everybody with a cardiac problem to the unit asap for further testing and if needed appropriate therapy. I have accompanied patients with unstable angina in a fully equipped ambulance and even there it was not fun.
I attended to several emergencies on BA and LH as a physician (with licenses in the US and Europe), and in the case of BA the captain followed my colleague and my advise to land asap (unfortunately we were in the middle of the atlantic, the next hours were not pleasant). And I was allowed into the flight deck to talk with the pilots and to medlink (who concurred with all our doings), who accidentally called me captain...
BTW, LH has a list of doctors who are registered via Miles and More and knows in advance if there is a physician on board - can be pediatrician, cardiologist or obstetrician, who knows. But any of these will have more medical knowledge than the crew and can judge the situation better.