After retiring, I moved back permanently to the UK a year ago from the US. I have dual citizenship and want to transfer my 401k funds from the US to the UK. I was informed I would need to fill out a W8 Ben form as I may be eligible for the Treaty tax rate. Is this accurate?

You should consult with an international tax expert on a face-to-face basis to have him research this carefully before submitting the W-8BEN.

There are variables associated with this that defy an off-the-cuff answer, and, quite frankly, I do not have the time to do the research on this without compensation.

I don't think you can file Form W-8BEN if you are a U.S. citizen.