Zune Pass has only very recently become available in countries outside of the US. The rollout has coincided with the launch of Windows Phone 7 around the world, as Windows Phone 7 has a lot of integrated features that link up with Zune.
The problems with launching "all-you-can-eat" music services like this around the world come down to the fact that laws on licensing, downloads, file copying and copyright vary around the world. Add that to the fact that even the big worldwide music publishing companies run different regions as separate 'child' companies and the company (Microsoft in this case) need to re-negotiate their deals with each music publishing company in each country to be able to offer their service there.
In your case I would suspect that your Live ID is associated with a US address and credit card, and the UK Zune service is blocking you as they're not legally allowed to offer anything under the UK agreement to US residents. A UK resident will see the same sort of thing if they try to subscribe to the US store.
You can either contact Microsoft and arrange to have your details set as a UK account, or create a brand new Live ID and purchase your Zune pass with a UK credit card that's linked to a UK address.