This is all a bit academic at least with respect to UK airlines.
The UK government has made it clear that international travel will not be permitted to return in any meaningful sense: the official line is that as long as there's a "threat of variants", a cautious approach will be taken (i.e. forever). In other words, the idea that foreign travel will ever return to 2019 levels is pure fantasy. The days of an annual two week foreign holiday or visiting friends and relatives abroad are gone permanently.