Catching American signal crayfish

Hi, recently there have been a few things on tele about catching american signal crafish so I reseached it and found that if you use a pole and line you dont have to have a licence. I do know that its an offence to catch our own crayfish and all that but
I just cannot find out the locations of where they are.  Are there any fishermen or fisherwommen who have had experience of these  , and know were they are. I live in south yorkshire but often go touring in North Yorkshire.  Any info would be greatly appreciated. 
Thanks