Would you be able to add Kings Road Veterinary Surgery at Biggin Hill to the list - they have done all treatments and surgeries for GBH (guinea pig, bunny, hamster) rescue in Beckenham and are highly recommended and experienced.
I personally recommend Mayow Veterinary Surgery, Mayow Road, Sydenham. They are not guinea pig specialists but have successfully performed tricky surgery on my pigs and more than once saved them from deaths door, and MADE appointments available for me even at staying open so I could dash after finding pig sick when coming home from work. In addition happy to liaise with other vets to quickly get meds they didn’t have readily available etc. And they are open 7 days a week - though sundays have a slight surcharge.