The answer to why there should be only one coronation is that what happens during it cannot be repeated. Those of Faith would say it is potentially blasphemous because it suggests the sacramental function has failed to take; those of no faith could argue that they don't hold with what it enacted, therefore better to have localised secular events.
Through its role and function, as well as by tradition and practice, the Commonwealth is centred on London: hence Westminster Abbey remains the Coronation Church for its Head. Each constituent part of the Commonwealth can hold its own Thanksgiving (both sacred and secular), which is why I stressed the importance of holding one at St Paul's, as well as St Giles'. The Honours of Scotland were presented to The Queen in 1953 and could be displayed in the future, as other countries display their own symbols during such events.
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