One way to structure your English literature dissertation is to research work by a single author. You can do this in many ways. For example, you could contrast work the author produced with historical events during his life -- is the tone of a book written before World War II markedly different from that of a book written after the war? If so, how is it different, and to what extent? How much of this can be attributed to the war itself?
Historical events help create the world in which authors write, and therefore also help shape their work. This means that a wealth of potential exists in looking at historical events and periods and contrasting them with work produced during that time. A historical topic will contrast work by multiple authors in relation to a historical event. For World War II, for example, you could choose a few books by each of a few authors, written before, during and after the war. You could analyze them for similarities and differences, then show to what extent the war caused these similarities and differences.
English literature dissertation topics can be extremely specific. You can analyze a single work, defining a question for it and trying to answer it through research and analysis. Ambiguities in work are excellent places to start. For example, Arthur C. Clarke's "2001: A Space Odyssey" has a variety of ambiguous notes, such as the failure of HAL 9000 and Bowman's ultimate destiny. You could write a paper that tries to answer the questions that the author left open-ended, shedding light on both the author and the work itself.
Literature is an expression of culture. Your dissertation can examine this fact by looking at pieces of work produced by a specific culture. Your work would analyze literature in terms of the culture that produced it. Which cultural values does it express most strongly, and which does it minimize? By doing this, you can find the most prevalent themes in a culture, which is valuable for academic discourse.