Hi Jamie!
I don't know how long your dissertation is, but if it is an undergrad or MA dissertation you may find that your topic is too vast for your 15000 words. I suggest that you REALLY narrow it down. The notion of "British theatre" or "social conditions" of England can make your research infinite! Why don't you just pick something narrower? Like the notion of "cruelty", "violence" or "radical individualism"? Why don't you pick up a particular play, instead, as opposed to the total oeuvre of a playwright?
Best of luck!
h.