Your Ratcatcher is a UK version of an old Crossman base model. I can not even find that particular model offered for sale over here.
Instead, we get the pump up version that eliminates the CO2 cartridge and I believe it goes slightly over your power limit, though not by much. That is probably why you get the CO2 version, so they can reduce the power level.
The pump system eliminates most of the objection about being SHTF ready or tracking down CO2 cartridges post event.
It is also considerably cheaper than the model you have available. Ours would cost you 60 pounds and you get the water bottle and pack along with the rifle. They are ripping you off magnificently!
http://www.walmart.com/ip/Crosman-Doomsd...t/21311833