International carriage is given a wide definition. It covers any flights where the country you depart from and the country of your destination are countries which have signed the MC.
Passengers departing from the UK on a roundtrip beginning and ending here would be covered by the Convention. One way trips to non-Convention countries would not be covered but as there are so few of those it is of little practical significance.
The UK and the EU have extended the same protection to domestic flights within the UK and the EU so a flight from Newcastle to Heathrow or Edinburgh to Stansted or Paris to Marseille would be covered.
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