i have been reading the warlord chronicles and I don't know why people are so terrified of the Brits here when they've been invaded and occupied so many times

Roman conquest (43 CE).

Anglo-Saxon migrations (5th–6th centuries).

Viking invasions and settlements (8th–11th centuries).

Norman conquest (1066).

Later medieval incursions: e.g., Welsh wars, Scottish wars, Irish campaigns, but these were more internal consolidation by England.

Spanish Armada (1588, failed).

English Civil Wars (17th c., internal).

Napoleonic threat (never materialized).

WWII German threat (Battle of Britain)