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Grade I listed buildings and structures are of exceptional, even international importance. There are over 6,000 in the country. Only 2.5% of listed buildings are Grade I listed.
In Scotland the classification is Category A
Index: Link

St Pancras Station is celebrated for its Victorian architecture, the Grade I listed building stands on Euston Road in St Pancras, London, between the British Library, King's Cross station and the Regent's Canal. It was opened in 1868 by the Midland Railway as the southern terminus of that company's Midland Main Line, arriving from the East Midlands and Yorkshire. At the time of opening, the arched Barlow train shed was the largest single-span roof in the world.

Link