There are several reference sources which have been used to build the underlying database.
Returns of Election Expenses
These have been published after every general election since 1868, with the single exception of 1918, and list the names the candidates (or their agents) gave, together with the number of votes. These are assembled from the individual declarations and returns made by each constituency's returning officer. Since 2005, expenses returns have been posted on the Electoral Commission's website. These are not always 100% correct.
F W S Craig / Parliamentary Research Services
Craig published four definitive volumes of British Parliamentary Election Results, 1832-1885, 1885-1918, 1918-1950 and 1950-1974. A supplementary volume containing results between 1974 and 1983 was also published. He also wrote many other reference works relating to parliamentary elections, including summaries, manifestos, constituency boundaries and "Most Gracious Speeches". Craig died in 1989, and his records were passed to Thrasher and Rawlings in Plymouth.
The History of Parliament
The History of Parliament project was a huge project to document the history of every constituency and MP since the late 14th century. The project split the history into periods - 1386-1421, 1509-58, 1558-1603, 1604-29, 1660-90, 1690-1715, 1715-54, 1754-90, 1790-1820 and 1820-32 - and each period had at least one volume detailing election results and giving constituency profiles as well as general political contexts for the period, and then a number of volumes containing biographical information for every MP. There are still gaps in the historical coverage, but it is complete from 1660. The project has also started working on a set of biographies for all members of the House of Lords from 1604. The project moved online around 2005 - see below.
Parliamentary Election Results in Ireland, 1801-1922 - B M Walker (Royal Irish Academy, 1978) This book contains details of all Irish constituencies since 1801. F W S Craig's volumes of election results did not include results from Ireland until partition in 1922, when results from Northern Ireland were included.
McCalmont's Parliamentary Poll Book of All Elections 1832-1918 - F H McCalmont (Harvester Press, 1971) This book is in three parts - 1832-1885, 1885-Jan 1910, Dec 1910. It also includes all Irish results and has most of the polling dates.
Dod's Parliamentary Companions
The Times Guides to the Commons
Published after each General Election, the Times Guides contain biographical information about most candidates who stood for election.
The Parliamentary History of the Principality of Wales - W R Williams (Brecknock, 1895) This fascinating book displays its Victorian credentials, and is well-researched for its period. Some of its contents do conflict with other sources, but it holds details of elections from when Wales started to return MPs in the sixteenth century.
The Parliaments of England - Henry Stooks Smith (Simpkin, Marshall & Co, 1844-49) These books are interesting little volumes, and display the essence of early Victorian record-keeping which can look slightly quirky to modern eyes.
The BBC/ITN Guide to the New Parliamentary Constituencies - Parliamentary Research Services, 1983 This book attempted to project what the 1979 General Election results would have been on the boundaries implemented for the 1983 General Election. It also contained details of which areas the new constituencies comprised, and linked them to the old constituencies. This turned out to be a one-off in terms of detail.
Boundary Commission Reports
The Boundary Commissions - there are separate ones for England, Scotland, Wales and Northern Ireland - report periodically with recommendations for new boundaries for parliamentary constituencies. Reviews were carried out and implemented in 1947 (First Review), 1969 (Second Review - not implemented until the February 1974 general election), 1982 (Third Review), 1994 (Fourth Review) and 2007 (Fifth Review). Each review details the geographical contents and boundaries of the recommended constituencies, with later reviews also including electorates. The Sixth Review of 2011 was cancelled in 2013, and the subsequent review of 2018 was also not implemented. Reviews from 2011 are online.
There are a number of websites which have been used to add to or verify data acquired from books. For the last decade, the importance of sources on the internet has somewhat superceded the production of reference books.
Wikipedia
There is a certain level of snobbery when it comes to citing Wikipedia as a source, but mostly it's accurate, taking it's material from the same published sources listed above. Occasionally some of the cross-referencing goes awry, particularly when it comes to links between election results and candidate details.
History of Parliament
The History of Parliament project has transferred all of the contents of its books online. Their website still takes the structure of the books, so if an MP stood in elections in 1747, 1754 and 1760, they would have two biographies, one in the 1715-1754 volumes and the other in the 1754-1790 volumes. Election results are similarly split between "volumes". Nevertheless it is a very useful resource, and contains a number of interesting and useful biographical snapshots.
Leigh Rayment
Leigh Rayment published a series of lists of peers of the realm and members of the House of Commons. Some of his naming of constituencies was a bit awry - well, he was Australian - and he loved adding notes of MPs or peers who met sticky ends, were caught in scandals (of their time), or who were involved in changing laws or precedents. Leigh died in 2018, which is when his website (understandably) stopped being updated. I have extracted the latest versions of the pages from the webarchive project, and am working to update them. You can find out more here.
Peerage.com
Driven by genealogy, the peerage.com website held basic information to determine relationships between peers and members of their families - although now it just seems to point to the latest "roll of the peerage" which is "slightly" less helpful.
BBC Election Results
The BBC's website has contained details for each General Election since 1997, and historic election results with associated news coverage is currently held going back to 2010. Each constituency has a separate page.
Work is ongoing to add links to biographical webpages to each candidate which has them. It will take some time before this is made public, but the intention is to be able to look at a person's story rather than just the bald facts.