Richie,
there's a fantastic book called 'Food for free' by Richard Mabey, it's a cracking book for would be foragers.
Go for it with the Crayfish trap, you'll be doing the indigenous fish population a favour. The last time I used mine I had a pile of crays and a very good brown trout in it!! The trout went back though as I was hoping to catch it again later on a rod. I have family in Oxford and my nephews spend plenty of time crayfishing. I think the licence issue is a bit of a farce, I suppose the main thing is to be able to recognise a signal from a native and to make sure you don'r release them.
The main thing with foraging, is not to take on too much at once. Get a good book, look at your surrounding area and then read up on what you would expect to find in your area. Then get out there and get your hands dirty