hello OP, I didn't see your previous thread but I didn't want your post to go unanswered.
I'm sorry that Rape Crisis weren't helpful to you.
There is a big difference between coping with everyday life after a traumatic event, and actively putting yourself back into a situation where you know you will feel very anxious and on hyper-alert all night.
Can I ask if you have seen your GP? Some of your post indicates that you may have PTSD as a result of your assault. This can be treated and overcome, but it will take time and treatment. Unfortunately mental health services have been cut all over :( but your GP may be able to get you better help.
Regarding your friends, are they aware of what's happened to you? Have you been totally open i.e. "I was raped on a night out" rather than using more comfortable language like "I was taken advantage of" which may not fully explain to them the real nature of what happened?
It does not sound like this environment of a hen night will be good for your mental health and right now YOU should be your top priority. Please don't let the "I've spent £150" sway you. You've spent that money already, so you can either be 150 down and completely miserable and re-traumatised at an event you don't want to be at, or 150 down and sitting at home comfortably, feeling safe.