Flights between London and a Swedish ski resort are available to book during February half term - clearly they are not yet full
It's the flights on dates which are not UK half term that are no longer available to book.
I am very dubious that late winter ski flights would be full on all weekends before and after half term... but during UK half term, there is space on the flights in both directions. Seems more likely that somebody at the airline wants to cancel flights with low booking numbers... but doesn't want to hand back the money for those cancelled flights