You've got no idea what you're talking about. The BBC is forced to be impartial by law.
Who do you think writes such laws?
The government. What is the government made up of? People! In particular, people are biased, greedy, and selfish. There is never any way they can maintain that law.
Even if those on air keep opinions impartial, the mere inclusion of shows tips the scales one way or another. Even commercials have their own bias. Or haven't you noticed that commercials can even push political candidates?