Have you told them who was driving?
Was it within 28 days?
And did you give them a name and address for the driver?
So they can realistically contact him?
Ok. How much information did you give them?
When was the fine issued?
That is a problem then I'm afraid.
It used to be possible for registered keepers to avoid liability by refusing to name the driver.
However, the Protection of Freedoms Act 2012 made a registered keeper liable if he does not disclose the name of the driver and his address within 28 days of the fine. The law actually says 'current address for service' so an address in Japan would not comply.
In those circumstances, you are liable in law.
Whether they will sue or not is another matter. Often they don't as it isn't cost effective.
Can I clarify anything for you?