Rissoides desmaresti is a fairly common subtidal squillid found around the UK. It occasionally occurs in large numbers. I can't tell from the photo if this is it, but that would be my guess.
The Lurker's Guide has a photo of one and a few links to other UK sites that are very useful.
Being that squillids are spearers and not smashers, all the precautions concerning the power of the strike are unwarranted. However, they can produce a fairly nasty wound, so I would be careful handling them.
Roy