As an English person who now lives almost on the other side of the world, yes Mumsnet is absolutely a British forum. Outside of UK hours it is virtually dead for a starter! As to whether it is predominantly English that's harder to say. Of course being British doesn't mean that you are white or just have British heritage, but the Japanese community in the UK is relatively small (only 40,000 or so) and I don't think that Japanese names are frequently discussed as an option here. I certainly was a bit surprised to see the names in the baby name listing, and would have asked about heritage too because of course it is relevant and not immaterial.