Today there is a DL9900 LGW-ATL which is carrying passengers booked on yesterday's cancelled DL11. Today's DL11 goes at 18:00 instead of 09:25. Not sure if this is how numbering usually works with international cancelled flights. My wife was scheduled on today's DL11 and got turned away at check in and told to come back at 9:00 as they were only checking in people from yesterday's flight. I looked up flight status via DL.com and fortunately checked by entering the airports and not just the flight number. This showed me the 9900 flight as well as the delayed DL11, with 9900 leaving at 12:00 and 11 leaving at 18:00. She called DL and got the last empty seat on 9900 (an aisle seat, amazingly enough).

Point of post is just that I knew that DL11 is only flight from LGW to ATL and normally would only search for status on the flight number. By searching by airports rather than flight number I also found the 9900 flight. When I went back and tried searching by DL11 it showed today's delayed, yesterday's cancelled. Nothing to suggest there was another flight with available seats. She had previously called DL when she got an phone alert to say her flight was delayed and agent did not provide any info about alternatives, just told her DL11 was delayed.