To use an ANA Star Alliance Airpass, I need to provide proof that I reside outside of Japan. What would constitute acceptable proof?

Context: I'm an American living and working in the UK, and I'm planning a vacation trip to Japan. I want to buy an Airpass for domestic flights within Japan.

My local travel agent can sell me an Airpass, but says that I'll have to provide the proof of foreign residence upon check-in in Japan. Sadly, neither they nor ANA's airpass webpage can say what would constitute proof of residence. Would a utility bill suffice? Or the UK work visa in my American passport?