I am glad that I could answer your question. What different vets are using also depends on what they have in stock at the clinic. Ivermectin/selamectin based brands are not prescription-only medications (POMs) with licensing for a specific UK brand for a species in addition to the product coming in various strengths.
In any case, I would recommend that you get hold of some F10 disinfectant concentrate because that is also effective against ringworm, where hygiene is as important (if not more) to get on top of the long-lived and highly contagious, species jumping spores. The concentrate lasts for ages and is ultimately a lot cheaper than buying diluted anti-bacterial (but not anti-fungal) disinfectant.
You can find our very detailed and very practical ringworm guide via the link in my first post if you need it. We are here for any further questions.
I am however very sorry for the situation you are finding yourself in. Unfortunately, anybody can call themselves a rescue or a breeder in this country with no licensing and control; and sadly the results can be accordingly when you fall for a backyard breeder with a side line in no longer wanted stock.
At least your two girls are lucky to have found a good and caring owner!
Here is information on the recourse you can take:
Rescues (Adoption and Dating), Shops, Breeders or Online? - What to consider when getting guinea pigs
The links are all part of our very useful New Owners resource which is there to help you learn what is normal and not and deal with the more commonly encountered unanticipated issues. Over 15 years on this lively forum with tens of thousands of questions and up to half a century of owner experience have gone into our guides.
You may want to bookmark the link, browse, read and re-read at need since you will take different things from some guides at different levels of experience:
Getting Started - Essential Information for New Owners
We are of course always here for any questions as well as a supportive community.