In fact, there's an argument that the in-built government majority in the Commons makes that far more of a powerless rubber-stamping body in normal times (recent events notwithstanding!)
Arguably Farage has been a "shadow" party leader in the UK for years. I wonder how differently things would have gone under a PR system where there were a few UKIP MPs off in one corner. As it was, they had no influence right up to the point where they "flipped" a large section of the Tory party into being pro-Brexit.
I also think that Farage shows that having a safe seat in the media is far more important than in Parliament for small parties; the national media have consistently given him far more coverage than comparable small or regional parties, because he's "good entertainment".