You need AUK's affiliate organisation number. It may be CUK will tell you elsewise you'll have to ask someone at AUK.
I have a number which I have just sent to someone to use. He has promised feedback on whether it works. If you are not in a hurry please wait a few days and then contact me on membership@audax.uk and I will be able to give you the number in confidence, if you want to try before that you take your chance!
That was I, my main Cycling Club is BC affiliated but not CUK and the local group I'm a member of is only just starting off with CUKs input.
Received a load of e-mails from them this morning, though not sure if that's an automated thing or if their office does something.
Interestingly their e-mailed document that says "You're a member" says I should receive the membership pack in 10 to 14 days, while the e-mail itself says 2 to 4 weeks
What convinced me was that it's covering anywhere in the world except the US and Canada and at £10m it's considerably more than what any of the cycle travel or bike insurance companies I found covered for. Getting more than just insurance for the £25 quid as well was the killer, better than paying 50 to 100 odd for something you're not actually wanting to get the benefit of.
Was speaking to someone about the MCofS's liability insurance in relation to their proposal to add a pretty pathetic life insurance to membership (doesn't pay out on death and pays out paltry amounts on injuiry) and apparently it's never paid out any claim...