Sorry but I think this is irrelevant, they are in the UK, so you have to treat anybody now in the UK as a potential carrier, if they have spent the last 2 weeks in London, just think of the number of people who have been to London for a reason in the last 2 weeks, then think of the number of people who they have been in contact with. Granted if they had spent the last weeks in a Lincolnshire village & never travelled outside of the village in that time but I find that highly unlikely.
This is just typical of the UK civil service, as soon as the Chinese announced it, let's face it for them to announce it means it was already serious, we should have stopped all flights.