The UK has a First-Past-the-Post system like the US, so the system tends naturally towards a two-party system as in the US. However, in the UK there is a sizeable third party at the national level (the Liberal Democrats) and a number of regional parties such as the Scottish National Party, Plaid Cymru (Welsh Nationalist), and Sinn Fein (Irish Nationalist) which are able to win seats by using their local strength. And there's also the Green party

I think the differences between Labour and the Tories have been largely diluted at the national level because of the poll-driven chase of the political centre that all Western democracies seem to have embarked in. In countries with proportional representation you see less of this, with the largest two parties engaging in this game but the rest of the parties stake out more well-defined platforms.

The introduction of Proportional Representation in regional (and European) elections has helped "third parties" in the UK, too, so I think people who differ too much from the party line just split and form a different party. For instance, the UK Independence Party seems like the rabidly anti-EU wing of the Tories. But bth the Tories and Labour have (at the level of national leadership) strayed to far from their core constituency and have left a huge number of disaffected supporters.

how do the people ensure the leader they get upholds the values of the majority and not that of some opportunistic wing of the ruling party?

I don't think that is a problem plaguing most western democracies right now. The thing is, if one of the two main parties has a leader that seems more opportunistic, it's likely they'll lose the swing voters in the centre. I think that's ultimately what's going to hurt David Cameron. He's even more opportunistic than Brown or Blair have ever been.

Can the last politician to go out the revolving door please turn the lights off?