I suspect OP means that he "turned up" late for his first segment, namely LON-NYC and when that was cancelled, his return to LON was cancelled as well.

Apparently OP chose not to check in until he "turned up" at the airport at approximately T-50, and we can only presume that by the time he made it to the head of the line, it was past T-45 and he was late. If he has facts to the contrary, that is a different issue.

Sorry, but these deadlines are published, are not "small print", are the type of thing which one would ask if one did not know and there is an easy solution, namely checking in online in the 24 hours preceding the flight.