£10k cash withdrawal - how can I make this happen?

I received my bonus recently and thought I’d tick off a bucket list item of mine, holding £10k cash. I have absolutely no good reason for this other than that I promised myself I would when I was growing up. Childish, I know, but I’m expecting an interrogation when I ask my HSBC branch as they’re apparently renown for the ‘intense questioning’. It’s likely they’ll deny my request if I phrase it to the teller the same way I have done here so if anyone has some better ways of wording it or reasons that don’t require an invoice then advice would be appreciated. At the end of the day it’s my money and shouldn’t need a reason to begin with, so how can I make this happen?