Hmmm... where to start on that one!
I hope one of our American contributors can comment on the Democrats and Republicans as, from Britain, our perception may not be accurate. For what it's worth, mine would be that there is a distinct shift in emphasis between the parties, but it that it is nowhere near as distinct as can be found in other countries, mainly because the majority of US voters don't want it that way. I actually think that system works pretty well on one level, in that in a great many cases Americans seem to vote for a particular candidate, and their positions on certain issues, rather than just because of their party. The nature of those parties, and the areas of their support has also changed significantly over time. Of course, both would deny being "pro War and support the American Empire and American Imperialism". Oddly enough, despite perceptions elsewhere in the world, none of things - even if true - are exactly vote winners.
As to British politics, it's currently effectively a three-party system in England, although there are nationalist parties in Wales and Scotland that are in the mix as well. Northern Ireland is really totally different, and far too complicated to explain here. Those parties are the Conservatives (who would be the closest to the Republicans) who currently form a coalition government with the Liberal Democrats who were traditionally a 'centre' party before moving to the left and now, seemingly back again. The third main party is Labour, who formed the last (Tony Blair, Gordon Brown) government. Labour was traditionally an out-and-out socialist party (although always democratic rather than revolutionary), although they have moved gradually more and more towards the centre and are probably now the closest equivalent to the US Democrats. The parallels are not that exact, though; as many British Conservative politicians would make happy Democrats as happy Republicans for example. Probably the main difference is that the social conservative element has nothing like the clout it seems to in the US.