Greek destinations are very different to Italian ones. Italy Is open to UK originating visitors - Greece currently is not, and won’t be in a week’s time. The UK flight ban lasts until 29th June, and the UK is unlikely to be included in the lifting of international arrivals on 1st July. BA probably doesn’t want to cancel the Santorini flights, but it doesn’t have any choice - the Greek government likely won’t let them fly, so the cancellation is out of their control.