It wont actually make much difference to them if you take out a private prosecution, if found guilty no individual will go to jail, they will be find guilty as a whole(RSPCA) not individually (each officer)and any court fines given wont be coming out of the officers pocket if they are sued. They area registered charity and funding for court cases/fines come from the fund.
It may make a difference if there were a number of cases going against them at the same time in different parts of the country, to show that they are abusing their powers.