Here in Norfolk, 'lay' is used for preference. For example, "He's now laying on his bed," or "Hev a lay-down; dew, yew'll feel better." I believe it's because the word 'lie' has such strong connotations of dishonesty and deception. The meaning of 'lie' is so powerful that people are reluctant to use it even when it would be correct to do so. The two forms, 'lay' and 'lie' have therefore been strictly segregated according to their meaning, and 'lay' preferred subconsciously for having no dishonesty attached to it. If one wants to accuse someone of not telling the truth, one would say,"Yer looooooiiiing!" or, "Doon't yew looooiiii ter me!" with the word stressed and prolonged enormously. So when one meets it in the innocuous, "Lie down.", it grates and jars. Just a theory of mine!