Need a licence for 'fixed engines' in England and Wales - not likely to be issued to individuals on an occasional basis these days. Contact the EA for more details.
There's some quite serious fines for interfering with the native white claw crayfish too... in the order of £5k - which is a pretty big ouch.
The simplest method for catching signals is to make a drop net out of an old bicycle wheel and an old angling keepnet/landing net. Weight the rim and the centre of the net and bait it with something meaty (dead fish, chicken carcass from yesterday's roast, onion sack of Bakers complete filched from the dog's cupboard), preferably well attached as they will carry it off if it's not tethered. Then sit and wait for some minutes - exactly how many depends on how many are in the water you're at - before hauling the net in and grabbing the crays before them escape. I caught 19 good fat ones from a Kennet carrier using the above method in about 30 minutes some years back.