I've done it all my life, off-and-on----I told somebody, recently: "I shall take my leave, then"! !! LOL It just comes automatically, sometimes----'course it doesn't help that I watch alot of British TV, either. I'm like someone else said: that Aspies are like tape recorders (or, something like that); meaning, I guess, that we record everything we hear, and regurgitate at a time that it doesn't make much sense, or is inappropriate, or whatever. That's why I never made fun of Madonna when she started sounding British because I experienced the same thing after living over there a couple of years.
The other thing is that I write in what now would be considered "Old English", I guess. When I was learning English, grammar, etc. in school, "alot", for instance was ONE word----as was "thankyou" (as a salutation: "Thankyou for going to the store for me"), and lots of other stuff. I guess old habits die hard; but, to ME, I see it as Americans MURDERING the English language, and I DON'T LIKE IT!! ! Don't get me wrong, I can speak slang, with the best of them----even on this site, there are forums where I will speak more casually----but, if I'm having a "serious" conversation, I won't use it, or, at least, very minimally.