Generally if you want to be better informed as to the possibility of a counterfactual, you should ask what factors lead to the OTL occurring. For example: what factors led to the abandonment of Britain by the Romans and was the collapse of Roman-British administration and culture a direct result of that?

This reduces the likelihood that your question will be construed as a request for speculation, as you are merely asking for information which is only coincidentally related to your counterfactual. You yourself will have to weigh up the probabilities based on the info you're given, because that is beyond the purview of this sub.