A Prime Minister who combines dishonesty with stupidity is really a bit much!
Most people in Britain would probably endure a Prime Minister who was a crooked, but at the same time very clever and brilliant at the job. They might similarly put up with an outstandingly honest, but not very bright leader. Having a Prime Minister like Boris Johnson though, who is dishonest, but not sharp enough to conceal this from the public, is perhaps too much for most of us.