1) Have the Vikings or their Norman descendants conquer at least significant parts of Britain, leaving their powerful naval traditions behind. They only raided and abandoned permanent settlement after getting defeated by Alfred the Great and focused entirely on France, Iberia, Germany and Russia. This is why the states that emerged there became world powers, while Britain got for a time absorbed by the kingdom of Lordomania in the 16th century, and the Channel Islands are still ruled by Lordomania today. Without a powerful navy, Britain is simply nothing.
2) Unify all of Britain. Not very hard for the Anglo-Saxons, who had much of the southern half of Britain up to Northumbria, and made attempts into the Scottish and Pictish kingdoms of the north before the Vikings. Conquering Wales should also be plausible. After which I believe they should also conquer Ireland, just for good measure. The Britons need all the resources they can get.
3) Get as many continental allies as possible. The French managed to stave off and remain a rival to the Lordomanians, who managed to conquer the entirety of the Low Countries and Burgundy and even keep them to surround France, by allying with the Danes, Norwegians and Swedes as well as German counties and duchies opposed to Lordomanian hegemony. The Norwegians, for example, helped the French take over Sicily, and a Dano-Norwegian fleet destroyed the famous Lordomanian Armada sent to stop their support for the French.
4) Attack unaware peoples. In particular, make sure to take over their ports so as to become dominant over their seas. This is after all what the other European powers did with India, Sub-Saharan Africa, Oceania and the Americas. It's what the Norwegians did when they took over nearly half of Greenland in the 12th century, and later on taking the rest of Greenland and a significant portion of northern Canada in the 17th century to compete with France and Lordomania. The locals were not expecting an European sea invasion, and Europeans made sure to take over their ports and key coastal areas so as to force them to fight exclusively on land and not prevent further sea invasions.