Reminds me of something: why on earth was the England NT so dire in the ate 70s/early 80s? The league topped the coefficients and Liverpool/Nottingham won the European Cup six years running.
I think it would be fair to say that Scotland in the 1970s was the nation producing the best of British talent in that period. If you had a British select team from roughly 1974-1986 I reckon the core of the team would have been Scots.
edit: Just checking the Home International Championships
1974 - Scotland 2-0 England Winners: Scotland
1975 - England 5-1 Scotland Winners: England
1976 - Scotland 2-1 England Winners: Scotland
1977 - England 1-2 Scotland Winners: Scotland (Wales 2nd place)
1978 - Scotland 0-1 England Winners: England
1979 - England 3-1 Scotland Winners: England
1980 - Scotland 0-2 England Winners: Northern Ireland
1981 - England 0-1 Scotland (No winners, Northern Ireland unable to complete fixtures due to Troubles, winners would have been Scotland or Wales)
1982 - Scotland 0-1 England Winners: England
1983 - England 2-0 Scotland Winners: England
1984 - Scotland 1-1 England Winners: Northern Ireland (Wales 2nd)
By the mid-late 80s England had recovered to become the dominant British force - there was the short lived Rous Cup between England and Scotland for 2 years - Scotland winning in 1985, England in 1986, and the competition between 1987 and 1989 featured a South American guest side - England won 2 and drew 1 of the 3 fixtures against Scotland.
However, certainly in the mid 70s Scotland were the premier British nation - qualified for 2 World Cups in 1974 and 1978, which England missed out on, and won 3 out of 4 games in that period as well.