"He wields approximately the same kind of power as the President in terms of making policy decisions, leading the government, and approving legislation (although the monarch has the final say-so)."

Well, yes and no. A British PM with a working majority in the Commons is a vastly more powerful political figure than any US president: there's a reason Lord Hailsham called the office an elected dictatorship, and it's only become more so since he said that back in the 60's.

"He/She is the party leader of the majority-elected party in the lower house of the UK legislature..."

Yes, but for those unfamiliar with the system, the question then becomes what happens if no one has no majority (a "hung parliament"). It happens far less in British-modeled "Westminster" parliamentary systems than on the Continent where you often have even more parties, but then you either have a very precarious minority government (as is essentially the situation right now in Canada), or multiple parties have to form a coalition. There hasn't been a coalition government in the UK since WWII, and a peacetime coalition since the 20's, but I think it's a more likely possibility after the upcoming election than it probably has been in decades: the Tories are regaining much of what they've lost since 1997, but aren't back quite enough to win it, especially given that much of their vote will just be "wasted" by getting stronger in seats they already hold.

Indeed, there are multiple recipes in results of this election for serious constitutional crises, especially in combination: think of the Tories winning the most votes but Labour still having the most seats, combine that with a hung parliament and/or combine that with the Tories getting a plurality of seats in England and Labour having to rely on Scottish and Welsh MP's for a majority even though they'd be voting on thngs for England that in their constituencies are matters for their own devolved legislatures rather than for Westminster. Could be a very interesting mess, I'm sure. ;-)