What's she like when she's on her other side? (I don't know if the video showed it, but I'm at work and it decided to stop playing!)
To me, it certainly looks neurological. It looks too extreme to be ear related to me, as she doesn't seem to be using her legs properly?
I don't know if anyone has suggested it, but living in Warwickshire, you're less than an hour away from a highly recommended guinea pig vet in Northampton
http://www.catandrabbit.co.uk/
I appreciate you want to give her a chance, but like you say, she hasn't improved on the antibiotics, so IMO the best chance you can give her is to book her in to see Simon on Monday and see if anything can be done.
In the meantime, keep her warm, keep getting food into her if she's not eating for herself, keep her hydrated. If she will tolerate being on her other side, it might be a good idea to switch sides every now and again.
I cannot stress enough how much she should see a vet again, ASAP really.