This will be hard because Britain and Europe is simply not in the same position as Japan and Asia.
1) Britain is a lot closer to Europe than Japan is to Korea/China.
2) Japan was fairly lucky that it was rarely threatened by Asia. Britain, on the other hand, can easily be invaded (and was many times) because of #1. So Britain has a lot more reason to intervene in European affairs just to insure their own safety.
3) Since the Norman Conquest, British lords have had extensive lands on the continent. If the King of England and his nobles have extensive lands in France, they aren't going to be isolationist.
4) Britain was part of the greater Medieval world. The Catholic Church is going to entangle Britain in European affairs even if the English nobility does not. Marriages and such because of #2 and #3 are going to bring British nobles into contact with the rest of Europe. Britain looks to the continent for much of its cultural, legal, and historical heritage.
5) Britain was heavily dependent on the European economy. English wool was needed for the textile industry of Flanders, and the export tax was a major revenue for the crown. Isolationism implies very low level of trade, and there is no incentive for that. The crown will want the revenue, and the wool traders want the money as well. Any attempt to isolate Britain will lead to lots of smuggling.
In order to have Britain be isolationist, you need to address most of these issues, and that will be difficult.
You probably need to have no Roman invasion so that Britain stays culturally separate from the larger part of the Roman Empire. Then you need a centralized authority to control the entire island and be strong enough to impose isolation in the front of all the commercial, military, and cultural reasons not to. This is hard to do over a long period of time, although it could be done in the short term. You need Britain to be strong enough to prevent any attempt at invasion from Europe, but not so strong that the powerful centralized government would ever want to risk invading the continent. Again, this is hard to do in the long run.
The fact is that the English Channel is not wide enough to allow for the level of isolation that Japan had which only really occurred during the Tokugawa Shogunate. Prior to Tokugawa, Japan was fairly open to cultural and trade relations. Japan got its written language, silk, Buddhism, and many other things from China. Hideyoshi even invaded Korea. The Portuguese introduced Christianity. Japan was far enough way to discourage all bust the most committed invaders as the Sea of Japan is a more formidable obstacle than the English Channel.
1) Britain is a lot closer to Europe than Japan is to Korea/China.
2) Japan was fairly lucky that it was rarely threatened by Asia. Britain, on the other hand, can easily be invaded (and was many times) because of #1. So Britain has a lot more reason to intervene in European affairs just to insure their own safety.
3) Since the Norman Conquest, British lords have had extensive lands on the continent. If the King of England and his nobles have extensive lands in France, they aren't going to be isolationist.
4) Britain was part of the greater Medieval world. The Catholic Church is going to entangle Britain in European affairs even if the English nobility does not. Marriages and such because of #2 and #3 are going to bring British nobles into contact with the rest of Europe. Britain looks to the continent for much of its cultural, legal, and historical heritage.
5) Britain was heavily dependent on the European economy. English wool was needed for the textile industry of Flanders, and the export tax was a major revenue for the crown. Isolationism implies very low level of trade, and there is no incentive for that. The crown will want the revenue, and the wool traders want the money as well. Any attempt to isolate Britain will lead to lots of smuggling.
In order to have Britain be isolationist, you need to address most of these issues, and that will be difficult.
You probably need to have no Roman invasion so that Britain stays culturally separate from the larger part of the Roman Empire. Then you need a centralized authority to control the entire island and be strong enough to impose isolation in the front of all the commercial, military, and cultural reasons not to. This is hard to do over a long period of time, although it could be done in the short term. You need Britain to be strong enough to prevent any attempt at invasion from Europe, but not so strong that the powerful centralized government would ever want to risk invading the continent. Again, this is hard to do in the long run.
The fact is that the English Channel is not wide enough to allow for the level of isolation that Japan had which only really occurred during the Tokugawa Shogunate. Prior to Tokugawa, Japan was fairly open to cultural and trade relations. Japan got its written language, silk, Buddhism, and many other things from China. Hideyoshi even invaded Korea. The Portuguese introduced Christianity. Japan was far enough way to discourage all bust the most committed invaders as the Sea of Japan is a more formidable obstacle than the English Channel.