IOTL the plague reached England in Spring 1349.
The Hundred Years War had begun in 1337.
The only way I can see something like the premise happening, baring ASBs, is:
- That the HYW is avoided, or delayed until well after the plague hits Europe.
- That maybe the period of most rapid expansion of the plague in Europe (1348) happens a bit slower, enough for doctored people to establish a pattern of spread in time, and they tell the King of England to to cut off every maritime contact with the continent.
- That he listens, and this is properly enforced through the whole of England (pretty big IF considering we are in the Middle Ages), with ships detained, turned back or burned right away before they make landfall, or with infected and suspects of infection being quarantined in sections of port cities etc. That this is also implemented in Ireland and imitated by Scotland so the plague does not find an alternate way to the islands pass the Channel.
Well, it seems just too complicated by the looks of it. Besides, didn't England import wheat from France at this time? Britain would be avoiding a plague (and that's supposing that it is successful at it) only to get famine for who knows how many years. After a while someone will ignore warnings, go to the continent to get grain and screw up the whole plan.
It would be more likely to see individual cities implementing quarantine plans and escaping unharmed like Milan IOTL. A kingdom or island-wide quarantine policy is probably too big to be enforced.