Perhaps my memory is failing, but doesn't speedway have a World Cup tournament now?
Scotland and Wales could barely scrape enough riders together for a team, whereas Ireland doesn't have any speedway riders (and are you talking about an all-Ireland, Northern Ireland or Republic of Ireland team). The point though is, that the FIM stipulates one representative team per national federation, and at the moment the ACU is the national federation for Great Britain.
I think you'll find that in those days the ACU represented the whole Commonwealth. Originally there was one Commonwealth team riding as Great Britain, but in the early-70s the ACU started staged a qualifying round between English, Scottish, Australian and New Zealand representative teams to determine which side would represent them in the WTC 'proper'.
There were separate England and Scottish teams in the World Pairs rounds, but I think liberties were often taken with national teams in that competition.