Hi Kate,
Thanks for the reply.
We have a warehouse in the UK and USA and the products sold on each site vary, so we need to keep them seperate. We want our efforts to be pushed towards SEO best practice, that is to say that the hosting we want regionally and also to merge the brands together (they're currently under two different brand/domain names).
The site we have operating in the US currently isn't doing great and is taking a lot of effort in terms of SEO, effort we could be putting towards the .com. If we bring the US site under the flag of the .com our hope is that it will benefit from the .com's authority and receive a boost (we would also 301 redirect all traffic from the previous domain as not to lose any juice).