Let the car's owner know what happened if you haven't yet. They will get the citation in the mail, and there is a process for them to follow when someone else is driving (link below). But right now just get in touch with the owner.

As to whether the city will bother going after a UK driver for a photo ticket, I have no idea. If they do, then take care of it one way or another like you normally would (if you believe you are guilty then just pay it).

Red Light Camera Citation | Superior Court of California - County of San Francisco

edit: Also if you do get it in the mail and want to just pay it, I would pay online and not the pay by mail route, just in case the mail gets delayed past the due by date. The SF traffic authorities have a reputation for being a bit draconian and not having a sense of humor.