In Children of Men, Did Key's helper Miriam alert Luke and the Fishes to where Jasper's house was? If not, how did Luke find Jasper's house?
In the movie Jasper was a old family friend of Theo and Julian. He knew their son, so it stands to reason Julian had told the group she led, some information about Jasper, or maybe she kept files on her office. Deducting they went there was easy.
There is no in movie explanation how they get there. In the book the story is very different, so I wouldn't look at it for an explanation.