This is a female Stag beetle that flew through the open patio and was resting on my kitchen floor... I hadn't seen her there and jumped out of my skin after she worriedly waved at me so that I wouldn't step on her :-)
Our biggest beetle in the UK her jaws are tiny compared to the males. They don't have long at this stage of their lives, having lived as a larvae for many years chewing up rotten logs they pupate and emerge in this form to reproduce. They survive in this form only a few weeks. this one I believe is a 'Lesser stag beetle' as she is all black.