Irresponsible and downright dangerous.
Yes, irresponsible and, if Heathrow continued operations regardless (highly unlikely), then potentially dangerous.
Having said that, Heathrow's protestations that it takes climate change seriously
"We agree with the need to act on climate change, but that requires us to work together constructively - not commit serious criminal offences just as hard-working people prepare to spend a well-earned holiday with their family and friends."
would carry rather more weight if the airport wasn't simultaneously fudging the statistics that it publishes on the environmental performance of airlines at LHR, such that they are regularly awarded impossibly high
"Fly Quiet & Green" scores of around 40% more, on average, than their performance actually merits under Heathrow's own rules.