Yeah animal border control is tight. Quarantine in excess of 6 months in a common affair with all warm blooded animals (mammals). Cold blooded on the other hand is pretty straight forward. Normally you can get straight through customs and border control within the guidelines and animal import duty. With that said, all relevant Cites paperwork must be valid, stamped and sealed however and for the vast majority of Amphibians, this does not apply. For your Cats, it would be very problematic so I can see why you never crossed onto our soil. The UK in general is full of red tape, does and don't... slightly confusing even to us British nationals. There is however a loop hole in our system that is still a very grey area (not many peeps know of this). If you are a British citizen and declare your imports for example frogs to be of food value (i.e. to eat them!!) you pay zero import duty and tax... knowingly that one would never eat them (or would hope this was never the case) unless crossing over the pond and into France where Frogs are eaten and classed as a delicacy.