stupid tax question (UK)

I might be OK as my previous employers luckily don't know the rules and sent me a scan of my P45 anyway. Fingers crossed.

hullo UK ilx, i am currently invoicing for a small number of US dollars for a freelance writing job from the UK

i was told i need to fill in a W-9 but this cant be right as i am not a US citizen -- the likely equivalent for non-US citizens seems to be a W-8BEN

naturally the guidance for filling in this (quite short!) form is nightmarishly long and baffling -- does anyone (a) have practical experience of this process or (b) have an up-to-date link which will talk me through it sensibly

QUESTION IN THE MEANTIME: am i correct in believing that line 6 in the W-8BEN (which requests "foreign tax identifying number"), i just put my UK Tax reference number (as supplied by HMRC)?