Labour is your typical big government leftist party similar to the democrats in this country. Libdems are actually a combination of two parties the Liberals and Social Democratic. The liberals were roughly equivalent to the Libertarian party in the US and the Social Democrats were basically socialists. A strange combination! The closest thing I could think of in the US would be if the Libertarian party and Ralph Naders group joined forces. They are general considered a centrist party. Although they are farther to the left than Labour on some issues and farther right than the conservatives on others.
Not really. The Social Democrats were a splinter group from Labour, but on the right wing of that party; who decided to go it alone as Labour veered sharply leftwards towards European-style Marxism after its 1979 defeat by Thatcher. The SDP were concensus centrists who briefly seemed likely to be a major new force, but merged with the Liberals when their star rapidly waned.