As noted, the Lib-Dems almost always capture a fairly large chunk of the vote but never see it rewarded with an amount of seats that does these achievements justice. When they receive more seats it's because they concentrate their campaigning on individual districts.
The USA has a similar problem and in fact have half a party less (two party system instead of 2.5), but it works better in practice. The reason, as it seems to me, is that congressmen are more beholden to their constituents than the GOP or the DNC. In the Netherlands, party discipline is stricter but we have proportional representation wich increases the viability of smaller parties. The UK combines constituency representation with centralized party discipline.
The Labour party has in the past argued for PR along with the Lib-Dems but has abandoned this idea when they seized the majority again. The only hope that I see for it in the UK is in the scenario that neither Labour or the Conservatives gain a majority and require at least tacit support of the Lib-Dems for running the government. I'm not very hopeful about that.
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