"To avoid total destruction, they (the British) challenged their conquerors (the Saxons) to battle, under the leadership of Ambrosius Aurelianus, perhaps the last of the Romans to survive...To them the Lord granted victory; for, from then on, sometimes our countrymen won victories, sometimes the enemy...This continued until the year of the siege of Badon Hill...” 1
That quote is basically all that is known about Ambrosius Aurelianus. It is possible he was the son of the Ambrosius who fought against Vortigern, but that is just a guess. Some have hypothesised that Ambrosius is the same man as Arthur, especially since Gildas mentions Badon Hill (Arthur’s greatest victory). However, there are several problems with this argument. First is the length of time. Although it is impossible to be sure about dates, it can be assumed Ambrosius took command a little after 460. The Battle of Badon Hill was probably not fought until at least 510, and Arthur lived for another 20 years beyond that. Ambrosius would have had to have been incredibly long lived. Also, no matter how you manipulate it, or what languages you use, the two names are just not the same.
The only real conclusion is that at some point, for reasons unknown, Arthur succeeded Ambrosius as leader of the British, and continued the constant struggle against the Saxon invaders.
Of those decades that followed immediately after 460, we know almost nothing of the British. We do, however, know a little bit about the Saxons, especially Allele the invader of Sussex and the first Bretwalda.
1) Quote taken from The Age of Arthur, John Morris, Phoenix, 2004