Do you have any gastro problems such as gastritis or a hiatal hernia or both? If you get these symptoms after eating it may be caused by gastro issues. Which can have symptoms similar to heart issues. If you look at gastritis and hiatal hernia forums people have similar reactions. Do you burp a lot after meals or after feeling faint?
This is a complete guess but I, on occasion, can inject insulin, eat something and then feel nauseous, have an elevated heart rate, burp a lot, feel shaky and weak. I hope you get to the bottom of it, I know how NHS doctors like to treat people like they are medical mysteries and do no investigation at all.