When you report, the police will initially only ask you a few basic questions, like the time and place of the attack, some of the details of the attack and a description of the attacker. Thepolice will take only brief notes at this stage.
They will make another date to take a full statement from you, and will refer you to a specialist team dealing with sexual offences if one exists in your area. Every London borough has one of these, called a Sapphire Team.
If the rape is recent, they will arrange for you to be examined by a police doctor.