Short answer. LHR has told all airlines, to stop selling tickets and cap capacity on LHR departures. So if you’re looking to “book” a flight it won’t let you. But it will let you book the inbound leg to LHR, as no capacity restrictions exist inbound that I’m aware of. I’d recommend you check the timetable section on BA.com.
Obviously people already booked will be able to travel, unless advised otherwise, and the aircraft will fly LHR-NCL-LHR as normal.