London
In terms of formal, sit-down restaurants with licenses, Wilton's dates from 1742 but has changed location several times.
Simpsons changed from a pub to a restaurant at some stage since 1757
Rules has stayed in place since 1798
The oldest pubs (that would invariably have served food) date from the 1200's - but cellars aside (which also date from that century), the oldest still in the same building is Spaniards Inn from 1585, that still serves food.
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