I didn't necessarily mean Striker. They've published various other comic strips in the past. Axa, Libby, Badlands... the only other one they can't use is Psycops, as that was also by Pete Nash.
Nope, I have no idea what the status of the other individual strips is but they certainly don't own the rights to Axa-her creator, Enrique Romero, does-or to Hagar, which is an American strip, so I'd imagine the same probably applies to all the rest. The Sun comic strips aren't generally work for hire as far as I know, so my best guess would be that once a strip is cancelled and its creator is no longer under contract that strip is then out of their hands.