This is the Leach’s Hermit Crab (Pagurus prideaux)...
"Leach’s hermit crabs and cloak anemones almost never appear without the other." If you are interested please read this amusing article on the relationship between these two species:
''Pagurus prideaux'' is a species of hermit crab in the family Paguridae. It is found in shallow waters off the northwest coast of Europe and usually lives symbiotically with the sea anemone ''Adamsia palliata''.