I can find no evidence on the internet that any such requirement exists.
The UK government (in common with many others) appears to require Advanced Passenger Information (API) for arrivals (and, as of 2015, departures) by ferry, rail or air. However there appears to be no deadline for when this is provided other than the arrival or departure of the vehicle in question. Most airlines and rail providers say they will take this information either in advance (their preference) or at the point of check in.
The UK's API program previously seems to have gone by the name "e-Borders", although that scheme is now defunct.
The 72 hours requirement is presumably on the part of your airline. I can give no guarantees, but I strongly suspect that if you have failed to provide the data with that much notice, that they will tkae it from you wheneer possible and pass it on to the relevant authorities.