Crust (2003)

Bill, an ex-boxer, is a failure in life but has no idea what to do about it -- until he meets local dodgy businessman Hamid 'Anything-U-Need' Choudhury. Hamid convinces Bill that there is serious money to be made in man vs animal boxing bouts. Bill cashes in his life savings to buy two meters of living, breathing, armor-plated giant mutant mantis shrimp with a smashing right hook, and sets off for London to fulfill his dreams. Along for the ride are his best friend Steve, ex-condom-factory employee and amateur boxer, Steve's highly-strung girlfriend Shaz, and a misunderstood, club-clawed crustacean who is not as stupid as it seems...