If you go by airplane, you can get to London in one day.
Literal
Airplane [by-で] go [if-ば] one-day [in-で] London [to-へ] can-go.
~ば marks a general conditional: 'if you X, then Y.' 行ける is the potential form of 行く. Two different uses of で in one sentence: instrumental (飛行機で = 'by airplane') and temporal scope (一日で = 'in one day').