Will a UK SNES adapter even fit? The US ones dont fit in a Genesis/Master System/NES/PCE.
And you only need a step down if you buy Japanese cables/adapters. I'd just recommend that since you'd be getting cables designed for the actual system, and then step down your countries voltage for it.
That one site that was linked to has Japanese adapters that work, AND stepdown transformers, so its got what you need. (You also need the step down since Japanese adapters have a different connector that wont fit in a europlug)
I'd still be skeptical about putting a UK power adapter into a Japanese system. But thats just me.