Richard Bridge:
In "U" circles in the UK one should say "Not at all", much as one NEVER replies to "How do you do" with ANYTHING other than "How do you do"

I haven't heard the expression "U" (and, by implication, "non-U") for years. Are they still used as class-defined-by-language terms? ("lavatory" - not "toilet"; "napkin" - not "serviette", etc.)

Alan Ross and Nancy Mitford had a lot to answer for... :-)