In theory it is down to BA, as the airline that has changed its schedule, to reaccommodate you. There is no need for them to have U availability and they can even accommodate you on another airline. Example, I had a flight on British Midland to GYD that went tech and returned to LHR - they reaccommodated me (at my insistence) on the next flight to GYD - operated by Azal with whom BD had no alliance or codeshare.
[URL unfurl="true"]https://www.caa.co.uk/passengers/resolving-travel-problems/delays-and-cancellations/cancellations/[/URL]
