Confidence: It might be this.
Notes: OK, going out on a limb here, but I thought I'd try out the new RES Staphylinid key... This beetle was abundant on a rocky shore around the tideline. So, antennae unclubbed, insertions visible from above and well in front of eyes = Staphylininae. Side border of pronotum with a reflexed 'flange' (not v. easy to see in the pic), elytra meet squarely & first antennal segment not v. long, pronotum without random even punctures, no transverse band on elytra, and pronotum narrower towards base = genus Cafius. Then, no yellow stripe on sides of elytra, plus pronotum with more than 3 larger punctures (inc. the one near the front), plus location on Dorset/Devon border = C. fucicola (C. cicatrosus is further east and only has 2 or 3 larger punctures). Any thoughts?