As far as I am aware, it’s people who began their trip in the U.K. who are now worried about beating the quarantine on return.
https://www.britishairways.com/en-gb...exible-bookingIf your destination changes to red or amber
If you have booked a flight, and are travelling between 1 July and 30 September 2021 to a country which is added to the amber or red list while you are at your destination, we will waive any fare difference so you can rebook your return journey to an earlier date if necessary at no additional cost, subject to availability.