Successful invasions of Britain resulting in the conquest of the country
AD 43 Aulus Plautius subdues Britannia for Rome
449 Traditional date of the landing of Hengist and Horsa and the beginning of the Anglo-Saxon conquest of Britain
865 Danish invasion establishes the Danelaw as an independent set of kingdoms; eventually re-conquered by the descendants of Alfred the Great
1016 Sweyn Forkbeard conquers England; his son Canute ascends the throne
1066 William of Normandy defeats Harald Godwinson at Hastings and is crowned King of England
1485 Henry Tudor lands at Milford Haven, gathers an army and kills Richard III at the Battle of Shrewsbury, becoming Henry VII
1688 William of Orange lands at Tor Bay with an army, proceeds to overthrow James VII and II, becoming William III
One could also note the return of royal exiles who overthrew the monarch in various civil wars and coups. These are not quite the landing of an army from over the sea, even if they were successful.