The rules that the UK applies to its citizens around Covid is a matter for the UK to decide - be it strict or liberal. However, air travel at UK airports is based heavily on people travelling between the UK and other countries - this can only work if other countries make it relatively easy for people from the UK to enter their countries, i.e. no bans on flights from the UK, quarantine on entry or PCR test needed in advance.
Air travel from UK airports will revive only when other countries make it easy to enter their country from the UK - and that will not happen until the number of cases in the UK falls significantly