General Music Resources
The standard English-language encylopedia of music is Grove Music Online, which includes articles from the 8th edition of Grove’s Dictionary of Music and Musicians along with material from other Oxford University Press music reference tools. Grove Music Online includes articles on countries, cities, instruments, and a wide range of people (composers, performers, instrument builders, publishers) as well as on musical forms and techniques. It is a subscription publication but is available through many public libraries, universities and colleges.
Information on British musical personalities, not directly linked to music, is available through the Oxford Dictionary of National Biography, which may also have more detailed information than Grove in the case of lesser-known musical figures. The DNB is also a subscription publication, available through public, university and college libraries.
The 10-volume Biographical Dictionary of Actors, Actresses, Musicians, Dancers, Managers & Other Stage Personnel in London 1660-1800, ed. by Philip H. Highfill, Jr., Kalman A. Burnim, and Edward A. Langhans (Carbondale & Edwardsville: Southern Illinois University Press, 1973-1993) provides a wealth of information about theatrical personalities associated with stage music in domestic collections.
The International Music Score Library Project (ISMLP) is a free online library of public domain scores, supported by a large user group who supply scans and information. It allows browsing by time period and region, as well as by composer. It can be used to source music in cases where you have information about music in a historic property but no extant scores.
Allen Scott's Sourcebook for Research in Music (3rd ed., Bloomington: Indiana University Press, 2015) is an exhaustive index of print and online resources on virtually any music topic - useful if you need detailed information about existing research in a particular area.
RILM Abstracts of Music Literature is among several wide-ranging databases maintained by RILM (Repertoire International de Littérature Musicale), an international and multilingual body for the documentation and dissemination of musical research. It can be used to search for articles and books relevant to your topic. It is a subscription service, available from research and university libraries.