Obtaining a licence to trap signal crayfish is free of charge and is a formality but you do need the landowners permission. The relevant form (FR2)can be downloaded here,
http://www.efishbusiness.co.uk/formsandguides/FR2v051.pdf
Please be aware it is a legal requirement to check ANY traps at least once every 24 hours. On my local river I can fill a trap very quickly and often drop one in whilst I walk the dog and collect it 15 - 30 minutes later with up to 30 signals in it!
Signals make great eating and my favourite method is to keep them live in fresh clean water (pref' oxygenated and filtered or changed regularly) for a minimum 24 hours then dropped live into salted boiling water for 5 minutes. Be careful doing this as sometimes the flapping tail can flick boiling water over you! When cooled remove the meat from the tails and claws and fry gently in garlic butter for a few minutes.