Many people do not wish to report immediately, but decide after a while that they want to do so. This is perfectly acceptable and there are steps you can take to make this easier.
You can go to Safe Place any time after an assault and report and complete anonymous intelligence which lets the police know that the event has happened but discloses nothing about you
If you do not wish to see anyone, information on this Rape Crisis website states that it is possible to preserve some evidence yourself. They recommend putting your clothes (including e.g. sanitary pad) in a clean bag in the freezer can help preserve evidence, as can brushing your teeth and then freezing the toothbrush. (http://www.hertsrapecrisis.org.uk/gettinghelp.html)