Something horribly traumatic has happened to you, where you were drugged and unable to stop it. Your body remembers some of it, your mind snippets.
You are still in the period of shock that happens after such a massive trauma. This is why you feel unable to act. You are sort of frozen with fear. Gently, you need to sort of rouse yourself to act in more active way...you will need to let someone know what happened to you. Your friend who you rang for help at midnight (that's what you were doing) or someone else close to you? The GP might help you.
Someone needs to know so that they can then guide you to where you need to go next. But pick someone who is kind and reasonable, otherwise they might minimise it and try to get you to 'forget' or 'move on'.
The only way you are going to move forward is to somehow get justice. The best way is to go to the authorities. I know you are too frightened at the moment. But later on, when you are ready to move out of this phase of shock, you will want to take action. Just in case, keep any evidence you may still have. The jeans and knickers, the record of the phonecall. Even if you've washed things, put them all in a bag together to use at a later date if you decide to.
At the moment, you don't want to tell anyone. Your next step, when you're ready, is to tell someone you can trust to react in a kind reasonable way. Then together go and get these bastards through the courts.
I would be very surprised if there wasn't some sort of record of assault with these people. They told you they were swingers, but what they did wasn't consential. It might be you have the bit of evidence needed.
But I am not surprised you don't feel able to go there just yet. You are doing all the right things regarding the GP. Just make up that bag of stuff to use at a later date and consider someone you can trust who will help you through this.
You can go to a Sexual Assault Referral Centre (SARC) and your GP should give you the details. They'll advise you. It's sad that the rape crisis lady was so awful. Was she pushing for you to report it? Maybe she's become a bit desensitised to it all.