Many Americans find an English accent quite attractive and attribute a higher value to something from Britain. But this isn't news to you is it?
A current successful ad campaign is by a car insurance company Geico which uses a talking gecko with an Australian/British accent. But I'll let the little guy speak for himself via YouTube http://youtu.be/acCfnwTpdxU
OF course car insurance and health an fitness are different markets with different end users. For example this is a 100% online weight loss program . It appeals to both men and women htttp://www.7minuteworkout.com This program is global and is a US/Canadian venture.
Here's my point:
All marketing focuses on getting customers. The two main questions are : What do you customers buy? And why are you unique? Is there something uniquely British to the product or weight loss process that would differentiate it to consumers? Has anyone recommended testing whether you need to use an American voice over ?
These are all branding questions and from my 10+ years at Ogilvy & Mather NY, we have learned that they are to be carefully considered.
Without research, marketers are guessing at these answers. An ad agency would recommend doing some testing first: focus groups, polling, or social media.
However, with that all said when you have your final answer if you do need "Americanization" Roger is on the right track and from what I can gather, between his suggestions, and what Sue has offered, you should be fine.
Good luck on your project. I am available to discuss and provide additional insight.