First, you need to handfeed your guinea pigs. Secondly, get Osteocare and give her 1 ml in the morning and 1 ml in the evening for 3 days. Give it orally using a 1ml syring (no needle). Dragging the hind legs can be a sign of paralysis, which is very rare. Peter Gurney found that Osteocare can cure the problem and I've used it successfully on 2 guinea pigs in the last 20 years. Osteocare has calcium, magnesium, zinc, and vitamin D in the proper amounts. Scientific research into hind leg paralysis in guinea pigs is scarce and the cause for this condition is unknown. Osteocare is safe and you want to get the original formula. Hand-feeding will also help get rid of soft poo because a guinea pig with hind leg paralysis might not be eating enough hay. Look for Oxbow Critical Care because it is the top product available. You can also ask the pharmacist if there is a anti-diarhea medicine for dogs and cats containing Attapulgite and pectin, (or similar ingredients). The one I buy is a tablet and I give a guinea pig 1/8 of a tablet twice a day. Finally, since you aren't too happy with your veterinarian, ask a local guinea pig rescue or look up a breeder in the UK guinea pig association to recommend a vet near you. As for the association breeders, these are people who raise purebred to show and they love guinea pigs their guinea pigs as much as you do. They are not unscrupulous backyard breeders!. It is the very rare situation where a breeder does not take good care of guinea pigs and the last occurrence I knew of was many years ago in Australia. Some of the breeders are judges. You can find most of them on Facebook, so if you find one near you, send him or her a FB message or call on your phone. I know some of them personally and most of them have lots of experience with very good knowledge about how to care for sick or injured guinea pigs.
BUT FIRST GET THE OSTEOCARE!