Let's see:
-British Romance was weaker than the ancestors of Italian, French and Spanish since a lot of the local people in the countryside didn't speak it, much moreso than on the continent.
-Non-Latin British languages were weaker than continental Latin dialects because they had less prestige, were not associated with cities, administration and religion and were not uniform.
While the presence of different languages might explain the disappearance to some extent I don't think it is sufficient. After all both the Basque and the Brittany language survived despite also not being associated with the Roman upper class. France was also bilingual with both Celtic and Roman languages being spoken, but here the Germanic invasion simply lead to the extinction of Celtic while the Roman language persisted.
-Germanic conquest was muuuuch slower than on the continent so the Germanic invaders didn't bite off more than they could assimilate.
How was the conquest of England slower? It is generally assumed to have started in 440 when the Germanic mercenaries rebelled against the Romans. The first invasion of France took place thirty years earlier in 406, and other incursions had taken place since the 380s. The invasions of Italy begun at latest in 455 (sacking of Rome) and lasted for at least a century (until the invasion of the longobards in northern Italy). If anything the conquest of England seems to have been faster.
At the end of the fourth century England was as urbanised as the rest of the Roman Empire. The invasions and the collapse of long distance trade brought about a severe de-urbanisation, but I'm not aware that it was more severe than in France. Even if the English deurbanisation was particularly severe (explaining the different development of the language), this raises the questions why it was exceptionally severe.
-Many Germanic people already there.
If you are refering to the Germanic mercenaries living in England, such mercenaries were also common in continental Europe and often formed the nucleus of a Germanic state.
-More contact with the Germanic homeland.
This might explain the survival of the Italian language as Italy is separated by the Alps, but France also had a lot of contact with the Germanic homeland which was just over the Rhine, yet it did not take a Germanic language. I find it difficult to imagine that contact across a river was less often than contact across a sea.
-Some stuff I've read says that a lot of the continental Germanic armies were pretty diverge grab bags of mercenaries.
And the Germanic people invading England consisted out of Jutes, Angles, Saxons and Frisians, hardly a monolithic group.
That assumes that the native population remains; lowland Britain seems different, with migration to Brittany etc
DNA testing has shown that only about 30% of the genome is of anglo-saxon origin (e.g.
http://www.bbc.com/news/science-environment-35344663), so a significant part of the pre-conquest population must have remained.