I think its the 3 rows in front and 3 rows behind the infected person that are notified.
not sure if that changes in areas close to the toilets where there may be more footfall
it could be they only developed symptoms a few days after they arrived back in the UK and tested 5 or 6 days after their return, hence why you are hearing after 7 days.
If so they would probably have tested negative if any test on arrival had been available.