They set up requirements different from art.5. UCTA does not require previous advertising or an invitation to trade. This seems to be irreconcilable with the convention's aim of harmonisation. However, a court is always free to apply its domestic mandatory rules. Contract contains clause which would be void under the UCTA 1977.
Case is brought before an English court.
Assume that requirements of art.5 are not fulfilled, because there was no previous advertising. UCTA 1977, the consumer retains the protection of that Act. Contract as in previous example with French supplier. Action is brought in France. It was first discussed in 1979 and preceded by several drafts 17 Latin American states as well as the US and Canada are parties to the Convention.