So there are two ways a prime minister can lose his office. His government can "fall" and there be a new election. Or his party can boot him out, and a new PM be chosen by the party. I don't know the method for that, but I think we'll learn it in this book.
One more difference: when someone runs for Parliament, he doesn't have to (and usually doesn't) live in the area that he will represent. He gets "in" with party members, and they find a nice "seat" for him to run for. This explains how all these members of Parliament, even those from working class and poor district,all come rom the same small pool of the elite. As we will see, many of them have never seen their district until they run to represent it.
When people vote for their member, they know they are also voting on who will be the Prime minister. So presumably, they'll vote for a member they don't like to get the PM and politics they want. But if the PARTY doesn't like a person, they can refuse to let them run for that seat next time. Thus, his party can have a stronger hold on a politician than his constituents.
And of course, this hold is important for the party since if their members oppose them their government will "fall" and they will have to struggle to get a majority of seats in the next election.