What if English evolved from Brythonic instead of Germanic?

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This does not necessarily mean that what I said regarding the Norman scenario is false. Small amounts of Saxons came in several waves (auxiliaries for Roman troops, raiders, mercenaries employed to fight Pictish raiders, invaders, settlers) and that the Angles and the Jutes came later after seeing how well the Saxons were doing. Each wave reinforced the others numerically. To make a tiny Anglo-Saxon elite, you might want a one large wave instead. The case would resemble the invasion of France more, where a few giant waves of Germanic invaders took over. The Romance speakers fled to the countryside. The Celtic-speaking peasants started speaking Romance. The Frank's large numbers were irrelevant because they were stretched thin across France. Eventually, they started speaking the local Romance languages. Romance in France had the advantage of the fact that the society was largely Christian, the distance of Rome and the fact that the Franks, as Catholic allies of the Romans, felt that they needed to learn Latin. In Britain however, the Romance languages would probably give way to Celtic. Wales, Cornwall and Northern England only spoke Celtic languages. Rome is much farther away and separated by sea as well as land, the society was largely pagan, and the Anglo-Saxons, as random pagan barbarians with no contact with Rome, had no special feelings about Latin.
The single wave would have this affect because the following steps happen:

• Tribe seizes control over large area.
• Members of the tribe settle all over the territory. They are spread thin and loose their numerical advantage.
• No reinforcements mean that tribe is still spread thin.
• The language becomes a mark of the elite. (This is the last step for the real life Normans, but the rest applies here. Norman French died out in Britain because standard French was even more prestigious, and many Anglo-Saxons were elevated to the nobility)
• Conversion to Christian makes Latin the new mark of the elite.
• Without social status or numbers, language dies out.