They used to compete in the world cup as GB until 95 when they split them up (exception of 75 as an anomaly which admittedly does look confusing!). GB then toured or played in Tri Nations together as one collective in the early 2000s, splitting up again for World Cups.. But besides 2019 they've not had a GB side since 07, and 2019 was a flop so I don't know if we'll see them again!
Also, l am quite surprised South Africa is not a major force in Rugby League competition as the country is the strongest in Rugby Union code (ok, l know the All Blacks disagree).