800: Something happens.
Early 1100s: As a result of something happening, the marriage of Matilda of England and Holy Roman Emperor Henry V bears several healthy male children. Butterflies also make Henry more long lived.
1135: Henry I of England dies. Matilda, as his sole living heir, sails to England to take the throne with an accompaniment of Imperial knights. As OTL, Stephen of Blois contests this and tries to seize control. However, in TTL, Matilda has the full support of the Holy Roman Empire, and a stable line of succession behind her, making the barons less likely to support Stephen.
Stephen, however, does attempt a coup, and briefly manages to seize control of the kingdom.
1136: Henry V invades England with a view to reinstalling his wife as Queen of England. Despite the domestic difficulties of raising and army, a degree of support within England itself makes the invasion more successful.
1144: Matilda dies, and the throne passes to her son Henry. Henry becomes Henry II of England, and (after the death of his father) Henry VI of the Holy Roman Empire. The dynastic personal union between the two realms lasts for many years.