Another historical/social influence on the themes in English literature would be the change in England's role in the world.  During the late 19th and early 20th centuries, England is THE dominating world power with its strong sense of imperialism and its establishment of colonies and political influence all across the world.  After the First and even more-so after the Second World War, England's global reach was weakened.  This change in world veiw changed the literature.

As the world view became more focussed on England itself, you can see the rise in the interests of the people of England.  There are labor organizations rising in power.  Women are asserting their equal rights.   There is much more attention to social legislation and welfare concerns.  The country moved towards its more modern socialist state.  These concerns become the themes of their literature.

The romantic sentiments of the 19th century give way to the more serious concerns about social matters, political matters and involvement in two major world wars.

I would argue that 20th century British literature was highly influenced by Victorian literature in the 19th century.  Victorian literature brought gothic elements, romance, social justice and supernatural themes.  Contemporaries wanted to expand on or move beyond those elements.  As literature advanced, it built on that foundation.

Well, the 20th century literature is dominated by war with common themes of alienation, isolation and fragmentation. The century opened with the Boer War and continued through WWI, WWII, The Balkans, Korea, VietNam, The First Gulf, Granada and many many others we scarcely hear of. Consider the impact of WWI, "The war to end all wars" (until of course WWII!). Think about the destruction that could be accomplished by bombing cities and towns without ever facing the foe as in previous wars. Whole towns were destroyed, families uprooted, every one you knew gone.

And when talking about British Literature, we have to factor in Ireland which of course was war torn for most of the 20th century. Look at the poetry and short stories expecially that were produced and you will find the common theme of feeling alone. Much of the writing is marked by deep psychological trauma. Not happy stuff, for sure.

I think there are many influences which all contributed to the development of 20th century British literature. I’m not sure that there’s one influence that stands out more than the rest. Two that come to mind immediately are James Joyce and Virginia Woolf.

Although James Joyce is Irish, he is from the region and stands out among Modernist prose writers and might be considered the most innovative writer of the 20th century. He exemplified a lot of the Modernist tendencies including intertextuality, free verse or stream of consciousness and played with language to such an extent that his work characterized ambiguity in form and content. This would influence subsequent Modernist and Postmodernist writers because of the boundaries he broke with form and also because of the play of language which would influence the ambiguity of authors known for Existential, Absurdist, or even Post-structural themes.

The Modernist school had an influence on Britain, America and many other countries. Other authors and artists of this school include T.S. Elliot, Pablo Picasso, Bertolt Brecht, Ezra Pound, George Orwell, Salvador Dali and Aldous Huxley. Some of these artists and authors are not British but their influence certainly reached Britain.

Virginia Woolf’ work, in style and content, had a large influence on British literature in the 20th century and on literature and feminism in general. For example, her novel Orlando explored gender roles, the concept of time and how history constitutes individuals. It was a trans-gender novel and thus broke boundaries in cultural roles and boundaries of genre, mixing non-fiction with fiction.