How about the Labour Party leader Clement Attlee? This was the guy who began the criticism in Parliament against Chamberlain just after Chamberlain gave his call for unity speech. When Chamberlain realized his speech was poorly received, he decided to let Churchill speak before the House of Commons the next day in order to save Chamberlain's govt.
After that speech, Parliament voted and the result was 281 votes for adjournment and 200 votes against. Chamberlain's majority was just a mere 81, which was pretty slim indeed. Before Hitler's rampage, Chamberlain held a commanding 210 majority in Parliament. Two days later, he appealed to Laborites and Liberal leaders to join a national coalition government. But he was rebuffed. The Labour Party (which had 164 votes in Parliament) finally forced Chamberlain to resign.