Translation:British people speak English.
The issue isn't whether the statement is true or not. It's what's the best way to translate the sentence. 英国 originally referred to England (and so 英国人 = English(man/ person) because they were first used when the English arrived in China well before the U.K. was formed. So now 英国人 = both British person/ Briton/ Brit. AND also Englishman/ English person/ Englander.
While there are many other languages spoken by the people in the U.K., English is the official language.
Very few people outside of the UK would ever use the word British as far as i know, its antique. To an Australian, the phrase "English people speak English" makes more sense, and "English people speak British" makes almost as much sense as "British people speak English"