Encyclopedia and Background sources (sometimes referred to as "Reference sources") are extremely useful for providing authoritative overviews and background information on a topic. They will give you the terminology and the key names, dates and concepts that can be used when conducting online searches for scholarly articles, books, as well as for searching for primary sources and historical newspapers. Many reference sources also provide extensive bibliographies that list the important and influential scholarship on a given topic.
Collins Dictionary British History
Encompasses "all the key events and personalities in Britain’s history, from Roman times to the present day."
A dictionary of British history
Written by over 100 specialist contributors, this dictionary describes the people and events that have shaped and defined domestic, political, social, and cultural life in Britain since 55 BC.
Oxford Dictionary of National Biography - The national record of men and women who have shaped British history and culture, worldwide, from the Romans to the 21st century.
Reader's Guide to British History
"Contains A-Z entries on the history of Britain, from ancient and Roman Britain to the present day. Each entry lists 6-12 of the best-known books on the subject, then discusses those works in an essay of 800 to 1,000 words prepared by an expert in the field."