The BBC History magazine has compiled a list of the 10 worst Britons since 1000 AD.
According to magazine Editor, Dave Musgrove, the conflicting definitions of wickedness from historians involved, led to a pretty diverse list, which you can see below:
1900 to 2000: Oswald Mosley
1800 to 1900: Jack the Ripper
1700 to 1800: Duke of Cumberland (1721-1765)
1600 to 1700: Titus Oates
1500 to 1600: Sir Richard Rich (Lord Rich of Leighs)
1400 to 1500: Thomas Arundel (1353-1414)
1300 to 1400: Hugh Despenser (The Younger)
1200 to 1300: King John
1100 to 1200: Thomas Becket
Archbishop of Canterbury
1000 to 1100: Eadric Streona
I have to say, I have never heard of Eadric Streona, and it is bothersome that there are two Archbishops of Canterbury in the list! That said some people in this list deserve their place. Titus Oates for one, and Jack the Ripper and Sir Oswald Mosley.
That said I can think of a large no of other deeply unpleasant people in British history who would deserve a place here!