It would appear your only option is credit cards.
They tend to use credit card, cash on delivery, or bank transfer. Cash on delivery would only work for physical goods. The customer would probably run into problems with international bank transfers (they would probably have to subscribe to another service, such as Lloyd's or JP Bank). Credit card would be easiest as the CC company would handle the back end.
Barring that, you would probably have to set up a bank account in Japan to alleviate the international burden on bank transfers, and it should open up the possibility of CoD for physical goods.
By "UK roads" do you mean some kind of oval track?
Come on, the UK isn't that small.
Early modern: SI/metric system
Modern: Planck units
Post Modern: ???