You’ve got the details a little wrong. The original two were the Whigs and the Tories, as you say. The Whigs became the Liberals who became the modern day Liberal Democrats, who still exist but haven’t been in power outside of being a junior member of a coalition for a century. Tories became the Conservatives, who are still one of the major two and are regularly still called the Tories. There was a faction that broke away from the Whigs called the Liberal Unionists, who merged into the Conservatives, but they’re separate from the Liberals. Labour is not a successor to either of them, though they did make some strategic agreements with the Liberals early on. In the early 1900s, Labour replaced the Liberals as one of the two major parties.

It is still consistently a two-party system. One of the historic parties got replaced and there is a stronger presence for minor parties than there is in the states (see especially the SNP in the past decade and the Tory-LibDem coalition in 2010), but still a two-party system