Foraging forumagers

Loads of freshwater rivers have crayfish - you need to as for permission though -
http://www.environment-agency.gov.uk/homeandleisure/recreation/fishing/38051.aspx

As for specific areas - I know Dorset has some - ask the local fishermen wherever you are.

I really want to try it in Poland but I can't figure out the legal status of using traps for one and I'm not even sure of the distribution of signal crayfish so until my polish is better i am going to have to wait.

Stick a trap in the water and find out, signal crayfish are easy to distinguish from European, I live near Polish border and as far as I'm aware there are no Signals here, though there are plenty of lakes.

Never use the same trap in different waters, spreads fishy infections and possibly signal crayfish eggs.