Hi,
There is a cultural bias against live feeding over here. But the legal sitch is fairly straight forward;
It is not illegal to feed live mammals to your snakes - it can, in fact, be illegal not to if that is all your snake will eat and you allow it to cause suffering or harm to the reptile. Incidentally it is also illegal not to take your animal to a qualified vet if you know they require treatment.
It is illegal to buy any mammal before it is weaned ( or usually interpreted as six weeks of age in rodents ).
It is illegal to feed live for the purposes of entertainment or doing so in such a way it causes unnecessary stress or harm to either animal.
That's where the RSPCA or SSPCA would get involved.
It is worth noting however that the RSPCA and their inspectors have no more legal rights or powers than anyone else - think of them as being similar to the people volunteering in charity shops. You are absolutely within your rights to deny them access to your property or animals despite what they will try and convince you of.
The SSPCA ( the Scottish equivalent ) however do have more legal powers at their disposal. They also tend to know their bum from their elbow - not something the RSPCA has often demonstrated with reptiles.
dr del