Hi Net66,
As I read through your question my first impression was... No Waaaaay!
I wouldn't advice to mix things up. Just simple create a new brand for the US market. The reasons are the following:
1. When re-targeting an established website you can hurt your rankings in both UK and US results.
2. a .co.uk is a ccTLD which wont be very useful for a US targeting
3. for a better on-site optimization you will have to rewrite content to better fit the US English readers level (and there are major differences between US and UK English) - What happens to their established visitors?
4. if you reach to retarget somehow the .co.uk website, they will see a fall in the sales (because they will probably loose some ranks in the .co.uk and gain some new ones in the .com)
Ok. Now if you buy a new brand name for the US market:
1. You can develop from first stage an optimized website (information architecture, content, links, etc.)
2. You can have a brand name that is more catchy for US market.
3. They can run two different websites in the same time effectively. Until the US website comes up, they will have enough sales from UK markets.
I hope that helped,
Istvan