How did succession in Anglo Saxon England work?

Here is a list of the various successions of the monarchs of the house of Wessex beginning with the death of King Ecgberht who founded a new branch of the royal dynasty. The statistics of the different types of successions could be the basis of a theory about the succession in in later Anglo-Saxon England.

1. Aethelwulf (795/810-858), son of Ecberht, succeeds in 839. Age 29-44. King of Wessex. Son 1.

2. Aethelbald (835/840-860) Son of Aethelwulf. Appointed under king in 855 age 15-20 and co king in 856 age 16-21. Full king in 858 age 18-23. Son 2.

3. & 4. Aethelberht (830/35-865/66) son of Aethelwulf. Co King in Kent, Essex, Surrey, & Sussex in 858 age 23-28, King of Wessex in 860 age 25-30. Son 3 (858) Brother 1 (860).

5. Aethelred (844/47-871) Son of Aethelwulf. Age 18-22. Brother 2.

6. Alfred the Great (849-899) Son of Aethelwulf. Age 22. Brother 3. First brother to succeed when his predecessor had surviving sons.

7. Edward the Elder (872?-924). Son of Alfred. Age 27? Son 4.

8. Aethelstan (895?-939) Son of Edward. Age 29? First King of the English in 927. Son 5.

9. Edmund (921-946) Son of Edward. Age 18. Brother 4.

10. Eadred (924?-955) Son of Edward. Age 22? Brother 5. Second brother to succeed when his predecessor had surviving sons. Note that his cousin Turketul (d. 975) was the son of Aethelweard (880?-922) and thus probably years older than Eadred but was not made king.

http://fmg.ac/Projects/MedLands/ENGLAND,%20AngloSaxon%20&%20Danish%20Kings.htm#Alfreddied899A1

11. Eadwig (940?-959) Son of Edmund. Age 15? Nephew 1.

12. Edgar (943-975) Son of Edmund. Age 16? Brother 6.

13. Edward The Martyr (963?-978) Son of Edgar. Age 12? Son 6.

14. Aethelred the Unready (966?-1016) Son of Edgar. Age 12? Brother 7.

15. Swen Forkbeard (960?-1014) Foreign invader and usurper. Age 53. Usurper 1.

16. Aethelred the Unready (966?-1016) Son of Edgar. Age 48. Rightful king restored 1.

17. Edmund Ironside (990?-1016) Son of Aethelred the Unready. Age 26. Son 7.

18. Canute the Great (995?-1035) Son of Swen Forkbeard. Age 21? Son 8? or Usurper 2? or Rightful king restored 2?

19. Harald I Harefoot (1016/17-1040) Son of Canute the great. Age 18-19? Son 9? or Usurper 3?

20. Harthacnut (1018-1042) Son of Canute the great. Age 22. Brother 8? or Usurper 4?

21. Edward the Confessor (1005?-1066) Son of Aethelred the Unready. Age 37? Son 10 (son of Aethelred)? Brother (full brother of Edmund Ironside and Half brother on their mother's side of Harthacnut) 9? Rightful king restored 3?

So Edward the Confessor was the 21st successor of Ecgberht. The 21 successions include 7 to 10 sons, 7 to 9 brothers, 1 nephew, 1 to 4 usurpers, and 1 to 3 rightful kings restored. So the 21 successions have 17 to 27 examples of the 5 types, depending on how a particular succession is counted.

Note that only two of the brothers who succeeded had predecessors with surviving sons. So there are only two examples of older brothers being preferred to younger sons.

If the four Danish kings and Aethelred the Unready's restoration are ignored, there are 16 successions, 7 sons, 8 brothers, and 1 nephew. And only two of the 8 brothers succeeded a predecessor who had surviving sons.

see also: Did Edward the Confessor choose Harold Godwinson as his successor?2

https://historum.com/threads/heirs-of-anglo-saxon-kings-of-england.126138/3

1066 Succession Crisis4

Technically, at that time kings were decided upon by the Witenagemot (assembly). We're not sure how pro-forma that typically was, but this was the accepted way a new King gained their legitimacy as ruler.

No man can make himself king, but the people has the choice to choose as king whom they please; but after he is consecrated as king, he then has dominion over the people, and they cannot shake his yoke off their necks.

(Ælfric of Eynsham, 10th Century)

This means there was a certain elective element in the position, and thus at the absolute least, an ability to select someone else in the royal family if the dictates of strict primogeniture would have selected someone less suited for some reason.

In the particular case of you mentioned, there had been an agreement (compromise?) made during a previous Witenagemot that Alfred the Great's brother would become King, with Alfred his successor. This dully happened, but when Alfred died, this naturally led to it being debatable who should be next; the children of Alfred or of his brother.

In April 871 King Æthelred died and Alfred succeeded to the throne of Wessex and the burden of its defence, even though Æthelred left two under-age sons, Æthelhelm and Æthelwold. This was in accordance with the agreement that Æthelred and Alfred had made earlier that year in an assembly at "Swinbeorg". The brothers had agreed that whichever of them outlived the other would inherit the personal property that King Æthelwulf had left jointly to his sons in his will. The deceased's sons would receive only whatever property and riches their father had settled upon them, and whatever additional lands their uncle had acquired. The unstated premise was that the surviving brother would be king.

Perhaps the "original sin" in all of this was King Æthelwulf leaving everything to his sons jointly, rather than the typical medieval custom of leaving everything to the eldest. That basically left a succession time-bomb where everyone in that generation technically had equal claim to the throne. A Witenagemot settled it, but only by putting the crisis off to the next generation.

There is a lot that we do not fully understand about the details of the succession in Anglo Saxon England. Indeed, it seems quite likely that the role of the council ('witena ġemōt', or 'Witan', if you prefer) changed over time.

It seems certain that the council maintained some role in the succession process throughout the period. However, in general, the system of primogeniture does seem to have applied. We can be certain that by the tenth century, Ælfric of Eynsham was able to write:

No man can make himself king, but the people have the choice to select as king whom they please, but after he is consecrated as king, he then has dominion over the people and they cannot shake his yoke from their necks.

• Quoted in The Norman Conquest by Teresa Cole

The 'people' in this context were the council, or witena ġemōt.

We also know that the council appears to have had the power to remove a king, as illustrated in the cases of Sigeberht of Wessex and Alhred of Northumbria, but this power seems to have been seldom exercised.

If you can get hold of a copy, Chadwick's Studies on Anglo-Saxon Institutions contains most of what we know about the role of the council in the succession of Anglo Saxon kings, but be aware that a number of his conclusions and interpretations have been challenged by more recent authors.