[hellbender]

Actually, the oldest business in Britain is the royal family, which can be traced back to William the Bastard in 1066, and used taxation, cronyism, cruel & unusual punishments like drawing & quartering, and dynastic wars to keep itself in power until finally they were restrained by the Whigs and their American revolutionary offspring.

[SunkenCiv]

Well said.

[colorado tanker]

The royal estates were eventually forfeited to the government in the 18th Century. The business enterprise of the royal family is the Duchy of Lancaster, which Henry VII declared separate from the crown lands to be passed by inheritance to the Kings (and Queens) of England. It has assets, mostly real estate, all over England and Wales valued at about 383 million pounds.

So, you could date the Windsor family business from 1399, when Henry was crowned, although some take it back to the medieval foundation of the Duchy, before it united with the monarch.