I am not quite sure if there is anyone else here who thought about this issue before, but I'll give it a try.
I know I saw somewhere in Boston Med, where Mass General physicians collaborate with those in UK with something about organ transplants. I don't remember quite exactly, but from that episode I learned that practicing in the U.S. doesn't mean you cannot travel or collaborate with others overseas.
With that said, I think that you should improve your app as much as you can at this point and apply for MD only. There will be time you will be able to go back to the UK for whatever reason, and when that comes, I feel that you won't be happy with your DO degree at all.
Look into the requirements on practicing in UK vs. US and what should be done to "transfer" to another country to practice. Maybe that will be one of the first steps to figure out.
EDIT: just saw the post above. Looks like DO might be fine, after all.