Thought I would provide a brief update, as it seems this situation has been coming up in one or two new threads.
I bit the bullet and made a withdrawal from TIAA-CREF, wired directly to my UK-based sterling current account. A few observations:
The process was relatively painless and quick. I realised that the best way was to do this via calling them and speaking directly. They filled in the withdrawal information and sent me the form and I just filled in one or two blanks and signed it. I sent them (via a secure message on their Web platform) a scanned copy of the completed, signed withdrawal form, plus the wire information (they use the Global Pay system for wiring in either USD or local currency), plus yet another W-8BEN form, plus information to verify my UK bank account.
There was no transfer fee. I guess I should be thankful for small mercies.
The exchange rate seemed pretty decent. From what I could tell, it was about 1.5 cents above the mid-rate (the amount was a little over $13k net after tax withholding - see below).
As expected, they withheld 30%, Grrrrr.
I managed to get in under the wire (before end of the tax year), so I plan to submit a 1040-NR as soon as possible to retrieve the $5k or so they withheld.
Re: the above, can I ask whether I can submit my tax return any time from now? I printed off the 1040-NR and plan to remind myself what a 1040 looks like and what documentation I will need. TIAA-CREF did not really provide much of an official statement of the tax withheld - all I have currently is a screenshot under the "Income Summary" section of my account page that shows a deduction of ca $5.6k. It also says on the same Web page "Tax Information: View your tax statements to see the actual income amount distributed.", but at the moment there are no tax reporting forms in my account documents page. Do I need to wait until a tax reporting document appears or will the IRS know that TIAA-CREF withheld 30%?
I am hoping that once I have been through the whole process once, it will be easier after that, except for the IRS having 30% of my money, interest-free, until the tax year ends.