Hello, I am Dr. B, a licensed veterinarian and I would like to help you with Billy today.
First, I must note that if you are in the UK, then there should be a vet available even if this is a bank holiday. While you may not be in a part of the country where the vets have normal office hours today, the veterinary licensing board requires vets to offer emergency care for all patients outside normal working hours. This means they must have have a contingency plans for days and situations like this. This means that if you ring the practice, they will likely have a message to direct you on how to contact their emergency service. The emergency service could be provided by one of their vets who is on call or it may be that they contract out their emergency care to a local emergency practice. But in any case, Billy should have access to vet care today despite it being a holiday.
If you don't have a vet or this vet refuses to see you (which is inappropriate since they have a duty of care to help him in his time of need), then you can check RCVS register (LINK). or you can check here to find your local Vets Now (LINK) who are open all nights/weekends to find an emergency service that will see him.
The reason I am highlighting this is because rectal bleeding that is continuously seeping from guinea pig is a serious situation. This is because they don't have the same body reserves that bigger animals have and blood loss becomes significant much quicker for cavies. Therefore, if there is uncontrolled bleeding here, it needs to be addressed promptly to prevent serious consequences that could even be fatal.
In regards XXXXX XXXXX would cause rectal bleeding, there are a few considerations. But with this being dark blood continuously seeping from the rectum, tumors and ulcerated rectal polyps would be at the top of our list. If he had had diarrhea, then colitis or GI parasites would be a consideration but unfortunately less likely with the history you have provided. Trauma would be possible, but from your history this would have to occurred within the colon and the potential for isolated colonic trauma is unlikely.
Overall, I am not surprised that Billy is subdued since any volume of continuous blood loss is going to weaken him. And if it is continuously seeping, then it is imperative that it is addressed as soon as possible. I have to warn you that if there is a bleeding mass (be it cancer or polyp) present up in his GI, then his prognosis will be poor. That said, we cannot leave him suffering and bleeding out like this over the holiday.
Therefore, I would advise checking the RCVS register that I have linked to above to see what your veterinary options are in your location. You want to then phone them and contact their emergency service. They are obligated to see Billy if you ask them to. And in this case, I would strongly advise that you have him checked over to see if this bleeding can be staunched (and perhaps they can give fluids if his circulation is collapsing) or at least keep him from suffering with this.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
Remember that if you have any lingering questions or concerns, please reply so that we may continue our conversation. I will be happy to work with your further and do everything I can to provide you with the service you seek. Please remember to rate my answer when you are satisfied (with 4-5 stars or a happy face) so that I may receive credit for my assistance. Thank you & have a great day. : )