"an advantage of having both locations on one .com is that all links will help towards it, rather than getting less benefit by link-building to two completely separate sites"
I disagree.
There is no better way to geo-target your site than to adhere to the appropriate top-level domain. If your site has UK product, it belongs on a .co.uk, hosted on a server in the UK. That way, search engines know that you are specific to that region. Same goes for US, Canada, Mexico, Ireland, you name it. You want to be relevant, you need to take the right measures.
And link-building is just part of the game. Link building on two sites is nothing considering many people, including myself, operate 5+ websites on a regular basis, so that's not a valid reason/excuse. If you have thousands of US-based links pointing to your site, then your site will be ranked for US content. There is an equal amount of link building necessary whether you have US and UK content on one domain, or two, as either way you will still have to gather UK-based backlinks for your UK content, and US-based backlins for your US content. The benefits of two domains far outweigh any benefits of one as it's cleaner, and more specific to your product. You will show up in the correct SERP's (google.co.uk for UK, google.com for US).
It's just important to get it right the first time, that's all. Changing your mind 10 months from now to make a switch can take you down a few notches in the serp's.
If OP insists on staying on one domain, use a subdomain, and not a folder structure. Will at least give it some separation from the US content.