I know that The Best Offer has been shot in Italy and Central Europe, but the events shown in the movie seem to be taking place in an English-speaking country. I'm wondering if this was supposed to be United Kingdom. Is there anything in the movie or external sources that confirms or contradicts that?
This is one interpretation:
Shot all over northern Italy, Vienna and Prague, the film’s precise setting is deliberately left a blank. As in the director’s A Pure Formality and The Unknown Woman, the anonymous location “somewhere in Europe” makes it easier to accept an international cast speaking English.
See: this review. I cannot verify its authority but it seems plausible to me.