I finally spoke to customer services and she confirmed that yes you can leave and a note would be put on account so when you get final bill you can get back in contact to have termination fee waived. She kept saying if i didn't use 25gb then i didn't need to leave, tried explaining to her that by leaving i would be saving £143 and would just take a sim only deal for a tenner, thought O2 might have offered me a sim only but didn't, got my pac code ok.