LHR closed after the incident, because (I’m guessing here, but other sites/sources suggest this is the case):
– outbound aircraft took off from southern runway, leaving debris on the runway. As a safety precaution (cf. Concorde for potentials for Foreign Object Damage to aircraft), and to recover any debris, the runway was closed for inspection
– BA762 landed onto the northern runway, closing it
– Firecrews were dispatched to support the Mayday landing, and involved in the ‘incident’ (i.e. putting any fire out on BA762)
– As firecrews were dispatched to the incident, the ‘available’ fire crew capacity was decreased, so the airport cannot operate until requisite fire fighting capacity is available (to deal with any other incident occurs).
Thus the airport had to close until enough crews were released from dealing with the BA762 incident.