Here's why:
* Predicate Adjective: A predicate adjective modifies the subject of the sentence and follows a linking verb. Linking verbs connect the subject to a noun or adjective that renames or describes it.
* In this sentence:
* Subject: young British actor
* Linking Verb: was
* Predicate Adjective: popular
The sentence is saying that "the young British actor" was in a state of being "popular".