By the way, I just remembered something else that may be relevant to your situation. Back when I was starting to invest, I tried to open an account with Fidelity Japan, and they refused on the grounds that I am a US citizen, citing an SEC rule. This ticked me off enough to send an e-mail query to the SEC, who responded that since I don't live in the US, they would not necessarily consider me a "US person" for their purposes (note that this is different from the IRS's definition of "US person" -- the SEC is basically just saying here that they don't feel obligated to protect me), and that they would have no problem with me opening an account at Fidelity Japan. I relayed this to Fidelity Japan, and their response was, basically, "we don't care, the laws change all the time, and we have made the corporate decision that US citizens are too much of a hassle to deal with."
Of course, Fidelity in the US also refused to let me open an account, on the grounds that I am not a US resident. Joseph Heller would be proud.
Anyway, I don't know if Fidelity in the UK has the same policy as Fidelity Japan, but if they do, you might be out of luck anyway. You might want to ask them about this in advance.
Bpp