Both Jet2 and Jet2 Holidays are owned by Dart Group Plc. From what I was told, but with no facts to support the story, is that 2 B757s were leased from Allegiant and the contract stipulated that they remain in their livery. Jet2 management apparently liked the livery and, since the two airlines neither compete nor operate in the same part of the world, they decided to adopt an albeit slightly modified version for some of their other aircraft in the fleet promoting the Jet2 Holidays brand.