Do - Substitute verb

Do - Substitute Verb - (auxiliary verb + do) - In British English (but not American), do can be used alone as a substitute verb after an auxiliary verb. e.g

a). He smokes more than he used to - American english
He smokes more than he used to do.- British English

b). Do you think Phil will come? ~ He might.- American english
Do you think Phil will come? ~ He might do.- British English