Many thanks to those who flagged this: we were due to take the train to Gatwick but are now in an Uber. Our driver is on a group call with others who are near Gatwick, apparently 40 min queues to get near South terminal by car. We're going to try going to North and taking the shuttle train.

Have just received an email from BA:

We are aware of the security incident issue at Gatwick South Terminal and are in contact with the airport. We will provide a further update as soon as we are able. BA Euroflyer

So on that basis I think we have to try to get to the airport on time.