Not sure if there is a specific name for "udders" when they are in pup... Mammary glands, teats, nipples, call them what you will, I'm sure they'll understand what you mean.
You're doing the right thing - whatever your opinions of the RSPCA may be, sometimes it's the only way to deal with a problem like this. Taking the law into your own hands is all well & good, but case like this should be brought to everyone's attention.