The Directory of Royal Genealogy
http://www.dcs.hull.ac.uk/public/genealogy/royal/
This site, which is maintained by Brian Tomsett, of the Department of Computer Science, Hull University, contains a database containing the genealogy of the British Royal family and those linked to it via blood or marriage relationships. It claims to contain the genealogy of almost every ruling house in the western world because of the intermarriage that took place between them at some time or another. There are in excess of 30,000 individuals from the earliest times to the present in the database.
There appears to be no easy way of identifying people associated with Hertfordshire so for fun, I checked on Nell Gwynne, who is said to have connections with Tring Park (where I walk our dog) and found the following entry (links not copied):
Gywnne, Nell (Eleanor)
Born: 1650
Died: 1687
Father: Gwynne, Thomas
Associated with Stuart, Charles II, King of Britain
Child 1: Beauclerk, Charles, Duke of St Albans 1st, b 8 MAY 1670
Child 2: Beauclerk, James, Lord Beauclark, b 25 DEC 1671
A better known link, who spent part of her childhood at St Pauls Walden, Herts, is recorded as:
Bowes-Lyon, Elizabeth Angela Marguerite, Queen of Britain
Born: 4 AUG 1900, London, England
Baptised: 23 SEP 1900
Acceded: 12 MAY 1937, Westminster Abbey, London, England
Notes:The Complete Peerage vol.XIIpI,p.403,note a.Father: Bowes-Lyon, Claude George, Earl of Strathmore 14th, b. 14 MAR 1855
Mother: Cavendish-Bentinck, Cecilia Nina, Countess of Strathmore, b. 11 SEP 1862
Married 26 APR 1923, Westminster Abbey, London, England to Windsor, George VI (Albert), King of Britain
Child 1: Windsor, Elizabeth II Alexandra Mary, Queen of Britain, b. 21 APR 1926
Child 2: Windsor, Margaret Rose, Princess, b. 21 AUG 1930