It seems like financially it could be a big win for everyone because UK market rates are so much lower than US ones. There are inevitably a few complications with an international relationship, but the time zones are close enough to communicate reasonably when necessary, and so far the legal/tax/insurance side looks like there would be slight overheads but nothing prohibitive.
Someone from the US mailed me in the UK about this after I posted on the HN freelancer thread last month and I was speaking with someone else yesterday who had done it once before, so now I’m thinking there might be some very good opportunities in this area that are perhaps being overlooked because most of us haven’t done much trans-Atlantic networking.
Currently, you might get some joy using a cost argument as GBP is so poor. Generally, it doesn't really fly though; there are much cheaper places than the UK for general development. In my experience, there's more mileage from specialist experience. In my case finance but I could imagine it's true for, say, biochem or high performance vehicles too.
You might get questions or objections around timezones and taxes, so have answers ready. In general as long as the relationship clearly meets IRS rules that distinguish employees from contractors there’s no US tax liability for the hiring company.
Expect some customers to offer local (UK) rates rather than US rates. US companies outsource to India to save money. How much you can charge will depend on the skills and experience you bring and price relative to US freelancers.