I went to Oxford, and the arts undergrads there can get an MA OXON 7 years after matriculation (arts courses are generally 3 years, and you don't do anything other than send in a form to get the Master's). I doubt anyone confuses it with the MEng that I got.
Using the title is a poncey thing. The last time I remember someone insisting on it was Helmut Kohl, when he got pissed off at some journalist.