The Lucille Little Fine Arts Library has a large collection of acting editions of plays as well as books with monologues for all types of actors. Play scripts can be found on the second floor of the Little Library, under the "PR," "PS" and "PT" classifications. Monologues can be found under the call numbers beginning with PN 2080.
Many books that are useful for theatre research are located in the Young Library, so check locations carefully. For books that are not in the UK Libraries, you can place requests through InterLibrary Loan and pick up your books at either Little Fine Arts Library or Young Library.
3. Nexis Uni (giant newspaper database to search for reviews and popular-press articles about the theatre world)
Other potentially useful sites to search to find plays/inspiration, from Alexander Street Press: