Your links concern periods that predate even the European Coal and Steel Community!
Imperial Preference did not have to mean a hard stop to trading with The Commonwealth but EEC Preference (protection) did. In 1973, when the UK joined, the Common Agricultural Policy was already in place, with revenues from the high levies on food imports, as well as those from common external tariffs on industrial goods boosting the EEC’s own resources. At this time the UK was at an immediate disadvantage because it imported far more from non-EC countries.
Tony Blair Institute globalist nonsense. No thanks.