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All Hallows-by-the-Tower, also previously dedicated to St Mary the Virgin and sometimes known as All Hallows Barking, is an ancient Anglican church on Byward Street in the City of London, overlooking the Tower of London.
Founded in 675, it is one of the oldest churches in London, and contains inside a 7th-century Anglo-Saxon arch with recycled Roman tiles, the oldest surviving piece of church fabric in the city.
Grade I listed. Link
Wikipedia: Link

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