Here is a descent of Elizabeth Willoughby from Richard fitz Roy, the bastard son of King John of England:
1. Elizabeth Willoughby 1512-1562 = Sir Fulke Grevill, knight
2. Edward Willoughby = Margaret Nevill b. 1494/5
3. Richard Nevill, Lord Latimer ca. 1458-1530 = Anne Stafford
4. Sir Henry Nevill d. 1469 = Joan Bourchier
5. George Nevill, Lord Latimer = Elizabeth Beauchamp ca. 1417-1480
6. Richard Beauchamp, Earl of Warwick = Elizabeth Berkeley ca. 1386-1422
7. Thomas de Berkeley, Lord Berkeley 1352/3-1417 = Margaret de L'Isle
8. Maurice de Berkeley, Lord Berkeley ca. 1330-1368 = Elizabeth Despenser
9. Sir Thomas de Berkeley, Lord Berkeley ca. 1292-1361 = Margaret de Mortimer
10. Sir Maurice de Berkeley, Lord Berkeley ca. 1271-1326 = Eva la Zouche
11. Sir Thomas de Berkeley 1245-1321 = Joan de Ferrers
12. Sir Maurice de Berkeley = Isabel d. 1276
13. Richard fitz Roy = Rohese of Dover
14. John Lackland, King of England 1167-1216 = unknown mistress