Cut up by car pulling in front of me trying to make a turn onto slip road?

travelling down A34 in hants going ped the slip road for the A303 a car pulls in (10ft ) front of me and then comes to a virtual halt and tries to make then turning onto the slip road at the end of the hatchins 10-15 from the gr verge, I have no option but to collide with the vehicle as very busy in lane two with traffic. because I clipped the car that pulled in front of me my insurance say Im at fault. I had no stopping distance watsoever but the fault is all down to me. could anybody pse advise