There is no significant difference with the British system here: both are FPTP, and both could in theory keep even a 20% party outside Parliament. What makes Britain (or if I want to be more precise, England) different is two things.

One is the strong geographic differences in the vote, the strong local traditions behind one party or the other: a 15% national vote for LibDem could mean 50% vote in some districts.

The other is the reason why three parties with long-running traditions can exist: the historical two-party system of Liberals and Conservatives could be modified due to the dual impact of WWI and a massive expansion of the circle of voters, among whom Labour found a foothold.

_{*Lunatic*, n.
One whose delusions are out of fashion.}