If you're not sure whether you want to report to the police yet, you might choose to go directly to a Sexual Assault Referral Centre (SARC), if there is one in your area. It's good to take a friend or trusted person with you if you can.
At the SARC, you can have a forensic medical examination, as well as tests for Sexually Transmitted Infections (STIs) and pregnancy. The SARC should not pressure you into reporting to the police and they can store the results of the forensic examination (or evidence) until you make up your mind whether to report to the police or not. SARCs have specially trained professionals who can give you medical help and advice. They can also help and support you through your immediate trauma.
You can find your nearest SARC by calling the NHS 111 non-emergency service, speaking to your GP or the Accident & Emergency (A&E) department of your local hospital, or by visiting the NHS Choices website.