Could be a Dysdere Crocata. An attractive spider with reddish legs and cephalothorax and buffish brown abdoman - body length 12 mm.
Fangs are huge relative to body size and are opposable; used to good effect when capturing woodlice. Hides under stones during daytime. Widespread and fairly comman. Found in most habitats that support good numbers of woodlice.
http://www.butterfly-guide.co.uk/survival/spiders/pics/spid1 0.jpg
This is one of the few species in the UK that can bite humans. Although their venom is not dangerous to us, their bite can be painful due to the strength of their jaws and some people may have an allergic reaction to the venom. However, they are unlikely to bite.