A source said: "Mila is over for at least six months filming near London and Ashton has decided to head here too.
"They spend most of their time glued to each other's side."
Mila and Ashton - who got together following the actor's split from former wife Demi Moore - are said to have started looking at properties in the Marylebone area.
They hope to be settled in to their new pad by the time Mila, 28, stars working on Jupiter Ascending in the spring.
Ashton, 34, recently wrapped shooting his Steve Jobs biopic.