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Thanks you so much, it seems obvious now you've pointed it out. Thank you for your help!

It's really tough to predict. I think Eyepaq covered the basics well - you'll consolidate your link-juice, but you may harm your UK-specific ranking slightly. Whether the consolidation offsets the loss really depends a lot on your link profile, how Google treats your UK site (sometimes, English content, even across countries, is seen as partially duplicated), and your market focus. If 80%+ of your market is US-based, you're not ranking that well in the UK, and you don't have the resources to really push two domains, my gut reaction would be to favor consolidation. If you have two separate marketing efforts int he two countries and half or more of your sales are UK-based, then you'd be taking a real risk. I would check out the newish rel="alternate" hreflang="..." option: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=189077 It's specifically for helping same-language content rank correctly across regions, and Google seems to be pushing it publicly.

Thanks Jane, that's a great help!