Check out the BASC website - should awnser most of your questions there and i would suggest joining aswell
as for the catapult i used to have one a few years ago, great fun for squirrels etc but not as prficcient as the likes of an air rifle, rimfire rifle, lurcher or ferrets and nets
if your pretty skilled with it and enjoy using it then there's no need to change as long if your not in it for big bags because its perfectly legal... each man to their own
Alex