[znt]

Simple really:

British: http://translate.google.com/translate_tts?ie=UTF-8&q=Have%20...

American: http://translate.google.com/translate_tts?ie=UTF-8&q=Have%20...

I know they don't sound ideal, but this is the only distinction I can make via the resources I have.

[rabino]

I opened both links it two separate tabs (almost) at the same time and suddenly my office was flooded with a chorus of nice people hoping I'm having a good day.

You made my day, Sr.

Edit: Grammar.

[shibby]

I don't know anyone who sounds like the 'British' version I also doubt that that dialect would be preferred.

It would fall under the 'Queens English' or 'received pronunciation', which, in some studies, is considered less intelligent than Yorkshire dialect - http://www.theguardian.com/uk/2008/apr/04/6

[pbhjpbhj]

The "Have a wonderful" part is not uncommon but the "day" with long drawling "a" sounds super-strange to me.