They will doubtless have planned to schedule staff in from other airports (GLA/EDI/BRS/etc) and have hotels booked. So LHR won't be the only place with longer queues. Just a guess but there is nowhere else to get them. Some mgmt will go on the desks too.
By the way, it's not just the Paralympics that extend the date on, it's the arrival of all the officials, contestants, medical, support and govt hangers on from dozens of countries arrives in an increasing stream, that brings the date forward too. It's not just 'three weeks'.