That is not generally true. Countries with codified constitutions have special rules on constitutional amendments (examples include Australia and Switzerland wh

That is not generally true. Countries with codified constitutions have special rules on constitutional amendments (examples include Australia and Switzerland which require a public vote on constitutional amendments, and even the US requires 75% of state legislatures to approve amendments). Even New Zealand (which doesn't have a codified constitution) has passed laws -- such as the Electoral Act 1993 -- which restrict the power of future parliaments from passing certain legislation. The constitutional convention you mentioned basically only exists in any significant form in the UK, though the concept does exist in a lesser extent to procedural rules (standing orders) in Commonwealth-descended parliaments around the world.