Towing aircraft around LHR during the day isn’t an easy thing given how busy and congested the airfield is already. For those familiar with T3 long hauls and the delays that afternoon flights had, much of this was often down to towing. Some of those aircraft would arrive in morning but there was a 4hr (IIRC) limit to have the aircraft stay at an on pier gate. When this happened, everyone knew that there was an increased risk of a towing delay.