When Britain's maverick Social Democrats first surfaced on the political scene, there was skepticism they could ever make a go of it. Few pundits would disregard them today. Now that the Liberal Party had decided to join in electoral alliance with the Social Democratic Party, it is not impossible they will form the next British government after elections in 1984. Polls show some 40 percent of Britons favoring the centrist alliance, as against 31 percent for Labour and 24 percent for the Tories. If the political arithmetic holds up three years hence -- London, hang on to your hat!