Inheritance from the USA

Hi,
I have inherited an IRA (Individual Retirement Plan) from a relative in the US which will pay a lump sum to me as a beneficiary. I am attempting to understand the US Tax rules and regulations and whilst I believe I am almost there with the form and I'm aware that I need to complete W-8BEN, it is asking for a ITIN (Individual Taxpayer Identification Number). If I'm correct, if I don't submit the W-8BEN with this number then I will be subject to a 30% tax with-holding of the monies. However, from investigations, the UK are part of a Treaty with the US whereby 0% tax is payable. So,what I'm asking is a) has anyone inherited monies from the US? b) Submitted a W-8BEN? c) Submitted the form without a ITIN? d) Or d) Requested a ITIN? From my research to obtain a ITIN I would need to send my original passport to the US for validation which is something I am not prepared to do as may not have this returned for many weeks. Any help or advice would be gratefully received, please.