Rescue owner refusing treatment

Hi, I've been fostering a UK rescue for a while and I've been hearing things from others that the rescue re-ages cats to avoid paying for vaccinations and doesn't carry out checks on neutering but takes money for it anyway. In the foster group we are always encouraged to share medicine often out of date and sometimes meant for humans rather than cats and they don't want anyone to go to the vets unless the cats are at deaths door! Luckily I haven't yet had to help a sick cat but I don't feel it's right to do that and not get the animal seen by a vet when they are clearly suffering. I've also heard that some of the foster homes have upwards of 20 cats and are not kept in sanitary conditions and are overcrowded but I've not seen it with my own eyes. I do believe it's true though.

Recently a new fosterer has been chatting to me to get advice about some kittens she's been looking after. She said that the rescues owner has stopped her from accessing medical care for the kittens repeatedly and eventually the kittens got really sick and in pain and needed hospitalisation but the owner has removed the kittens out of the vets against the advice of the vet and the kittens are now suffering and are away from their mum.

I've not seen any of this but again I believe it's true and I'm shocked the vets have allowed someone to cause suffering to kittens that need to be hospitalised. I've had some doubts about the way things have been run for a while now: There's no contracts, there's no checks, nobody pushes for quarantine and they seem to keep getting more and more animals even when they say they are closed.

My gut tells me something is wrong and that the situation with the new fosterer and the sick kittens is not an isolated incident.

But as I've not witnessed a lot of this is there anything I can do?

Can the RSPCA investigate based on a second hand report as the fosterer is too scared to come forward? And should the vets have intervened and stopped the kittens from being discharged when they were suffering and needed hospitalisation?

I went into fostering to help animals and I don't want to stand by and let a rescue cause unnecessary suffering.

This is in England.