British Titles List -- Wikipedia list of extant and extinct titles dating from 1066-present. Includes the family name.
Book of Household Management by Isabella Beeton - Late 19th century, but chock full of advice on how to deal with servants and other household issues. Wonderful period recipes, as well as priceless info on what to do with rancid butter, etc...
Calendar-- Need to know what day Christmas fell on in 1814? The phase of the moon for a specific date in history? Check out this site. It can generate calendars for any year and any country.
Chambers' 1869 Book of Days -- detailed description of historic events, biographies of famous people, and customs and culture.
Climate History -- mean temperatures in Central England from 1659 to present. Figures are in celsius.
Even More Climate History-- rainfall in England and Wales from 1914 to present. Figures are in millimeters. Monthly minimum and maximums are calculated.
Consanguinity -- a table of marital relationships forbidden by the Church of England.
Country House Database -- a list of country houses in the British Isles from the late Middles Ages to ca. 1850. with an index of families.
Decoration of Houses by Edith Wharton-- Written in the late 19th century. Wonderful descriptions of walls and window treatments, floors and ceilings, halls, ballrooms and saloons. Trying to figure out what furniture is in your heroine's bedchamber? This is the place.
Domestic Medicine in 1785 by W. Buchan
19th Century Women and the Monthly Cycle -- by Virginia Mescher. How women coped.
Gaskell's Compendium of Forms-- Late 19th century but sure to be applicable earlier. Written form of address for letters of condolence and apology, dinner invitations, weddings and balls.
Great North Road -- history and construction of, geology, maps, distances between towns.
Horse/Traveling Facts -- Jean Ross Ewing/Julia Ross' excellent resource on the care and feeding of horses, plus how far they can travel.
How Much Is That? -- Calculate how much today's dollar or pound was worth in a specific year.
Inns of Old London-- alphabetical resource of information on the inns of London and other information on historic buildings in Britain.
Internation Dialects of English Archive -- hear different British accents.
Introduction to the House of Lords Ceremony --detailed account of the ceremony. It was changed based on the recommendations in this 1998 report.
Leigh's 1819 New Picture of London -- everything the Regency writer needs to know about life in London. From how provisions were supplied, to street indexes, places of worship and public buildings.
Letters from the Past -- provided by the Victorian Web. Information about postage costs during the Regency period, franking and who paid what. Many fine examples of period letters.
London Panoramas -- 360 degree views of select bridges, squares, parks, etc...
Making Fire with Flint & Steel -- the procedure for lighting fires in the days before Lucifer sticks.
Multimap -- find the distance between cities in Europe and Britain
1792-9 Map of London
Cary's 1818 Map of London
1827 Map of London
1859 Map of London